Energy Density Li-ion Batteries Research Articles

Reliable protocols for calculating the specific energy and energy density of Li-Ion batteries

The gravimetric and volumetric energy densities of lithium-ion batteries are key parameters for their implementation in real-life devices, yet to date, these values are documented differently both in academic and industrial reports, which makes the comparison of advances in this field challenging. This ambiguity stems from calculation methods that take into account different battery components, and since certain cell design parameters such as the mass loading and electrode density are often omitted, it is difficult to find out what assumptions are made in the calculations. Herein, we present calculation methods for the specific energy (gravimetric) and energy density (volumetric) that are appropriate for different stages of battery development: (i) material exploration, (ii) electrode design, and (iii) cell level engineering. These calculations help establishing a fair and robust method to compare energy metrics, and we highlight the calculation details that must be specified in reports to understand the reported figures unambiguously. Finally, we calculate how some key battery design parameters such as the mass loading, electrode density and N/P ratio affect the energy metrics.

Re-evaluating common electrolyte additives for high-voltage lithium ion batteries

Further increase in the specific energy/energy density of lithium ion batteries can be achieved via further increase of charge cell voltage. However, an enhanced electrode cross-talk, i.e., transition metal (TM) dissolution from cathode and deposition on the anode, drastically limits the cycle life, even leading to rollover failure. In this work, the commonly used film-forming electrolyte additives vinylene carbonate (VC), fluoroethylene carbonate (FEC), and lithium difluorophosphate (LiDFP) are thoroughly evaluated regarding their ability to suppress the issues originating from electrode cross-talk. Neither the VC- nor the FEC-containing electrolytes can suppress it, as evidenced by the presence of Ni, Co, and Mn on the graphite anode; although different for the FEC, the deposited TMs and intertwined Li deposits are homogeneously distributed in the presence of VC. Despite suppression of rollover failure in this manner, VC still cannot compete with LiDFP because LiDFP is able to complex TMs and provide TM-scavenging agents, i.e., PO 3 F − and PO 4 2− , thus, effectively suppressing electrode cross-talk in the first place and effectively preventing the concomitant failure cascade. TM-based electrode cross-talk causes Li dendrites and rollover failure Neither VC nor FEC can suppress electrode cross-talk At a minimum, VC promotes homogeneous TM deposition and suppresses rollover failure LiDFP is most effective because it suppresses electrode cross-talk via scavenging Further increases in energy density of Li-ion batteries can be realized by increasing the charge voltage. However, the enhanced transition-metal dissolution from the cathode and deposition on the anode causes severe safety and performance issues. Klein et al. show that scavenging of transition metals can ameliorate those issues.

Electrochemical and Structural Studies of LiNi0.85Co0.1Mn0.05O2, a Cathode Material for High Energy Density Li-Ion Batteries, Stabilized by Doping with Small Amounts of Tungsten

The specific capacity of Ni rich ) cathodes is higher as their Ni content is higher and can reach values up to 240 mAh g−1 (x →1) while being charged below 4.3 V vs Li. This property is very important since charging Li-ion batteries below this potential is not detrimental to anodic stability of the electrolyte solution. However, as the content of Ni increases, the electrochemical, structural and thermal stability decreases. Here we focus on which was stabilized by doping it with W by a simple solid-state method. This doping greatly improved the electrochemical, thermal and structural stability, although both X-ray diffraction and density functional theory (DFT) studies indicated negligible change in the structural parameters upon doping. For example, 96% capacity retention was achieved for doped cathodes after 120 cycles at 45 °C vs Li anodes, compared to 80% capacity retention for an undoped, reference cathode. Cross sectioning studies of cycled cathodes by SEM imaging of FIB-cut samples confirmed the correlation between structural and electrochemical stability. W-doping stabilizes the cathode material by reducing the formation of cracks and loss of the spherical shape of the active mass particles, upon prolonged cycling. Additionally, W-doping enhances the thermal stability of the cathode material. DFT based calculations indicated that doping this cathode material with tungsten, reduces the amount of Jahn-Teller active Ni3+ ions within the crystal lattice and introduces strong W-O bonds, which stabilize the active mass during repeated lithiation—lithiation cycling.

Interfacial Degradation in NMC811-Graphite Batteries during Extended Cycling

To improve the energy density in Li-ion batteries, development of cathode materials with higher capacities are required, where much of the current focus is on Ni-rich lithium nickel manganese oxides (NMC). However, higher Nickel content is typically accompanied by accelerated degradation and thus poor cycle life, with undesired reactions at the electrode-electrolyte interfaces strongly implicated. There have been numerous attempts to mitigate this, using various methods including surface coatings, structural doping and advanced synthesis techniques such as core-shell or concentration gradient particles with much lower nickel concentrations at their surface. Designing improved solutions, requires a deeper mechanistic understanding of the underlying interfacial processes involved in degradation and their relative contributions. In this study these reactions are investigated in LiNi0.8Mn0.1Co0.1O2–graphite cells cycled using LP57 electrolyte (1 M LiPF6 in EC:EMC, 3:7) for different number of cycles, providing information about how interfacial degradation proceeds. Electrochemical measurements are combined with post mortem techniques including surface sensitive soft X-ray absorption spectroscopy, and photoelectron spectroscopy measured with different excitation energies. Based on fitted differential voltage analysis (DVA) data from the cells it is shown how the initial capacity fading during the first 300 cycles is almost fully attributable to excessive electrolyte reduction exhibited as SEI thickening, whereas further cycling leads to a larger contribution to capacity fading from loss of active NMC material. The initial capacity fading due to electrolyte reduction can be correlated with more plating of manganese, as compared to nickel, onto the graphite electrode, see figure. However, after 600 cycles both nickel and manganese are found in similar concentrations within the SEI. The changing ratio between the amount of plated nickel and manganese with cycle number indicates that manganese on the NMC surface initially dissolves more easily than the nickel, but as the unstable manganese ions are dissolved more and more nickel will contribute to the transition metal dissolution. The nickel ions in the SEI are also found to change their chemical surrounding as the cycling proceeded, showing that they do not form stable species and are thus likely to contribute to continued breakdown of the SEI. We compare the observed evolution of the SEI that results from cross talk between the NMC and graphite electrodes, to the case where EC-free electrolytes are used and graphite is replaced with Li4Ti5O12 (LTO) such that contributions from electrolyte reduction at the anode are expected to be minimised [1,2].In addition to the side reactions at the graphite other important side reactions take place on the NMC, some of which are potentially initiated by the graphite electrode [3]. For instance it was found that the Li2CO3 impurities present on the NMC surface decompose throughout the cells’ cycle life rather than just during the initial cycles. Previous studies have shown how Li2CO3 coatings severely decrease the cycle life, and therefore the observations here show that decomposition related to Li2CO3 is a problem that does not disappear during the formation cycling. Instead minimizing the initial Li2CO3 formation is critical. The insights obtained here on the evolution of surface layers coupled with the individual electrochemical degradation of the electrodes will advance the understanding of the Ni-rich NMC’s capacity retention behaviour and inform future efforts to achieve longer cycle lifetimes.[1] Björklund, E., Göttlinger, M., Edström, K., Younesi, R. & Brandell, D. Sulfolane‐Based Ethylene Carbonate‐Free Electrolytes for LiNi0.6Mn0.2Co0.2O2−Li4Ti5O12 Batteries. Batter. Supercaps 3, 201– 207 (2020).[2] Björklund, E., Göttlinger, M., Edström, K., Brandell, D. & Younesi, R. Investigation of Dimethyl Carbonate and Propylene Carbonate Mixtures for LiNi0.6 Mn0.2Co0.2O2-Li4 Ti5 O12 Cells. ChemElectroChem 6, 3429–3436 (2019).[3] Björklund, E., Brandell, D., Hahlin, M., Edström, K. & Younesi, R. How the Negative Electrode Influences Interfacial and Electrochemical Properties of LiNi1/3Co1/3Mn1/3O2 Cathodes in Li-Ion Batteries. J. Electrochem. Soc. 164, A3054–A3059 (2017). Figure 1

Lithium Insoluble Interlayer Suppress Void Induced Dendrite Growth in Garnet (LLZTO) Electrolyte Based Li-Solid State Batteries

Lithium ion batteries (LIBs) have been in prevalence for over two decades. They find use in portable electronics, electric vehicles and also grid storage. The conventional Li-ion battery technology employs electrolytes that have flammable organic solvents as key constituents. In addition, these electrolytes are thermodynamically unstable at voltages beyond 4 V; stability at voltages beyond 4 V is essential for the development of high voltage and therefore, high energy density Li-ion batteries. Moreover, state-of-the-art lithium ion batteries use a graphitic anode, which has a low theoretical specific capacity of 372 mAh/g. Due to this low specific capacity, conventional LIBs are limited in scope with respect to their energy and power density. Lithium metal anodes with a theoretical specific capacity of 3860 mAh/g are ideal for the development of high energy density Li-ion batteries. However, Lithium metal when used with liquid electrolytes tends to form filament like structures called dendrites due to uneven deposition of Li metal leading to short battery lifetime and also possible battery fires.Solid state batteries (SSB’s) have been considered as a promising battery system to overcome the flaws of current LIB’s. The solid electrolyte (SE) employed in these battery systems are inflammable, eliminating any concerns regarding battery safety while also increasing battery’s operating temperature. A wide electrochemical stability of 0 to 5-6 V against Li+/Li and high sheer modulus opens up the window for use of high voltage cathodes and metallic lithium anode which otherwise cannot be employed in LIB’s. At cell level researchers have demonstrated an energy density of 400 Wh/kg which can further be increased to 800 Wh/kg, with a cycle life of 10,000 cycles, making them more than ideal candidate for EV’s, portable electronics and grid storage applications. LLZTO (Li 7 - x La 3 Zr 2 - x Ta x O 12 ) an inorganic ceramic electrolyte has attracted a lot of attention owing to its high ionic conductivity (10 - 3 S/cm) and stability against Li metal up to 6 V. However, dendrite growth at current densities as low as 100 µA/cm2 has been observed which has plagued their integration into all solid-state batteries. Improvements to the interface between lithium anode and the solid electrolyte via the use of metal interlayers which alloys with lithium and surface modification techniques, have been shown to mitigate dendrite growth at current densities exceeding 100 µA/cm2. When such interfacial improvements have been coupled with an external mechanical pressure of several hundred kilopascal, cells were shown to sustain higher current densities without any dendrite growth. Although such high stack pressures are impractical for battery development.In this work we have synthesized high ion conductivity pure cubic phase LLZTO and studied dendrite growth mechanism. We observe that voids at Li/LLZTO interface act as field concentrators and have prime role to play in nucleation of dendrites. We suggest that these voids might be inevitable owing to nature of this battery system and propose the use of electron conducting interlayers which have no solubility with lithium metal as way of suppressing voids and mitigating dendrite growth. Figure 1

3-Thiopheneboronic acid: an effective additive for regulation on electrode/electrolyte interphase of lithium metal battery with high-loading cathode

Lithium metal has been considered as a potential replacement for the commercialized graphite anode to further boost the energy density of Li-ion batteries. However, Li dendrite growth during Li plating/stripping causes safety concern and poor lifespan of Li metal batteries (LMBs). In addition, the deploy of Ni-rich high-loading LiNixCoyMn1-x-yO2 (NCM, 0.6 ≤ x ≤ 0.95) is also an efficient way for boosting energy density. However, the dissolution of transition metal ion and structure evolution during cell operation can leave an adverse effect on cell performances. Herein, dual-functional 3-thiopheneboronic acid (TB) additive is used to form a lithium borate-rich solid electrolyte interphase (SEI) on Li anode and an improved cathode electrolyte interphase (CEI) on Ni-rich cathode. The TB-induced SEI layer is conductive and stable, and thus beneficial to improving kinetic limitation of Li nucleation and obtain a uniform morphology of Li deposition. When the TB-protected Li metal anode matches the high-loading LiNi0.6Co0.2Mn0.2O2 (NMC622) cathode (13.65 mg cm−2), high initial capacities of 163.78 mAh g−1 (2.23 mAh cm−2) at 0.2 C and 164.82 mAh g−1 (2.25 mAh cm−2) at 0.5 C after activation are obtained, attributing to the effective SEI and stable CEI induced by TB. This strategy of introducing additives into LMB system with Ni-rich high-loading NMC cathode affords an emerging energy storage system to demonstrates the material engineering of batteries with very high energy density.

Li0.5PAA domains filled in porous sodium alginate skeleton: A 3D bicontinuous composite network binder to stabilize micro-silicon anode for high-performance lithium ion battery

An important strategy to improve energy density of Li-ion batteries is to substitute the traditional graphite anode by Si-based anode which is endowed with ultra-high theoretical specific capacity. However, the commercialization of Si anodes is hindered by its huge volume variation that results in electrode pulverization. In the current study, we fill up the pores of sodium alginate (SA) network with lithiated polyacrylic acid (LixPAA) to form a cross-linked bicontinuous composite network binder (b-Li0.5PAA@SA), in which the pores of the SA skeleton are dominated with the Li0.5PAA domains; within such composite the SA and Li0.5PAA interlock tightly each other via extensive interfacial ester bonding. The resulting b-Li0.5PAA@SA network binder can effectively buffer the volume variation of Si microparticles (m-Si) during repeating cycling and prevent pulverization of the electrode, which is evidenced by a cycling capacity of 2762 mAh g−1 in the 1st cycle and a retention of 1584 mAh g−1 after 150 cycles. As a comparison, the m-Si electrode with only SA binder suffers from fatal capacity degradation after merely 20 cycles under the same conditions. Moreover, since the Li0.5PAA domains are ionic conductive and significantly reduce the porosity of the SA network, the b-Li0.5PAA@SA network binder could also enable a stable solid electrolyte interphase (SEI) film and fast electron/ion transfer, leading to an enhanced rate capability of the m-Si anodes.

A review on passive cooling techniques for lithium-ion battery thermal management system of electric vehicle

Li ion batteries have gained popularity in electric vehicle application in past few years because of its high energy density and long-life cycle. The high energy density Li ion batteries are also generating a large amount of heat which needs to be dissipated for the stability, longevity of the battery and safety of vehicle. Thus, a battery thermal management system (BTMS) is very much essential for the battery system. The objective of BTMS is to maintain the cell temperature and thus improve life cycle of battery system. It has been reported that the battery pack has better thermal stability and lifetime when operated at a temperature range of 15 to 35 °C and maximum cell temperature difference of 5 °C. Among battery cooling techniques, passive approaches are considered as less complex and less expensive. In this context, a review on passive cooling methods for BTMS has been discussed in this paper. Subsequently, a summary of characteristic parameter for an efficient passive BTMS has been provided.

The role of O2 in O-redox cathodes for Li-ion batteries

The energy density of Li-ion batteries can be improved by storing charge at high voltages through the oxidation of oxide ions in the cathode material. However, oxidation of O2− triggers irreversible structural rearrangements in the bulk and an associated loss of the high voltage plateau, which is replaced by a lower discharge voltage, and a loss of O2 accompanied by densification at the surface. Here we consider various models for oxygen redox that are proposed in the literature and then describe a single unified model involving O2− oxidation to form O2, most of which is trapped in the bulk and the remainder of which evolves from the surface. The model extends the O2 formation and evolution at the surface, which is well known and well characterized, into the electrode particle bulk as caged O2 that can be reversibly reduced and oxidized. This converged understanding enables us to propose practical strategies to avoid oxygen-redox-induced instability and provide potential routes towards more reversible, high energy density Li-ion cathodes.

A Two-Step Oxidation Mechanism Controlled by Mn Migration Explains the First-Cycle Activation Behavior of Li2MnO3-Based Li-Excess Materials

Li-excess materials created by incorporating Li2MnO3 into traditional layered transition-metal oxides promise to substantially increase the energy density of Li-ion batteries. The precise redox mec...

Practical Approach to Enhance Compatibility in Silicon/Graphite Composites to Enable High-Capacity Li-Ion Battery Anodes

There is an urgent need to improve the energy density of Li-ion batteries to enable mass-market penetration of electric vehicles, grid-scale energy storage, and next-generation consumer electronics. Silicon–graphite composites are currently the most plausible anode material to overcome the capacity limit of graphite or poor cycling performance of silicon. One serious and unrecognized limitation to the use of the composite as an anode is the incompatibility of hydrophobic (natural) graphite with the hydrophilic Si, which adversely affects battery performance. Herein, we report a novel, practical approach to modify the graphite resulting in the formation of a hard carbon coating and graphene sheets that give rise to higher compatibility with Si nanoparticles in the composite. Electrochemical and battery testing of the composite (10 wt % Si) anode shows higher reversible capacity (10% at C/12 and 20% at C/2) than the composite with unmodified graphite reaching ∼600 mAh/g with 95% retention after 100 cycles. The enhanced battery performance is explained by the uniform distribution of Si nanoparticles at the modified graphite surface due to the presence of graphene conductive networks and a thin, oxygen-rich, amorphous carbon layer on the surface of graphite particles, as evidenced by transmission electron microscopy (TEM) images and X-ray photoelectron spectra (XPS). This work provides a new approach to prepare graphite compatible materials that can work with hydrophilic components other than silicon for various applications other than batteries.

Failure mode of thick cathodes for Li-ion batteries: Variation of state-of-charge along the electrode thickness direction

The construction of thick electrodes with high loading level is considered one of the promising approaches to improving the energy density of Li ion batteries because it can reduce the amounts of separators and current collectors in a cell. Thick electrodes, however, show poor rate capability and cycle performance compared to thin electrodes. Although the correlation between electrode thickness and electrochemical performance has been extensively investigated, the failure mechanism of thick electrodes has not yet been fully demonstrated. In this paper, a direct spatial analysis using the configuration of multi-stacked electrodes is introduced to investigate the failure mode of thick Li[Ni0.6Co0.2Mn0.2]O2 electrodes. Each layer of the multi-stacked electrodes is facilely separated after cycling, followed by performing various ex situ analyses on each layer, independently. Eventually, the degradation behavior of thick electrodes, such as the uneven state-of-charge (SOC) distribution along the electrode thickness direction during cycling, is demonstrated in terms of the deterioration of Li+ ion conduction in electrolytes. Thicker CEI layers are accumulated on the cathode surface closer to the outermost electrode during cycling, leading to pore clogging initiated at the outermost electrode. This results in a greater increase in ionic resistance closer to the outermost electrode during cycling. Changes in ionic resistance along the electrode thickness direction during cycling give rise to the uneven SOC distribution, thus resulting in the poor cycle performance of thick electrodes. Therefore, a particle size gradient electrode, in which the particle size gradually increases along the electrode thickness direction from the innermost to the outermost electrode, is introduced to mitigate the SOC inhomogeneity of thick electrodes during cycling. The optimized pore and particle size distributions along the electrode thickness direction suppress the uneven cell resistance during cycling, leading to the improved cycle performance of thick electrodes. These findings provide fundamental insights into strategies to design thick electrodes for high energy density Li ion batteries.

Theoretically Quantifying the Effect of Pre-Lithiation on Energy Density of Li-Ion Batteries

Pre-lithiation plays an increasingly significant role for high-energy Li-ion batteries (LIBs) since it can improve the energy density by compensating the Li loss during the initial cycle. The pre-lithiation related research so far has been focused on the development of materials and methods of pre-lithiation but has lacked theoretical and mathematical descriptions to illustrate the relationship between pre-lithiation and energy density. In this contribution, a series of mathematical formulas are derived to describe the gravimetric and volumetric energy densities of LIBs with pre-lithiation, by which the effects of the important parameters, e.g. the Coulombic efficiencies (CEs) of anode, the capacities of Li sources, etc., on the energy densities are well demonstrated. Then, the developed theory and mathematical formulas are applied to practical LIB systems, i.e. the cell using Li nickel manganese cobalt oxide (NMC) as a cathode and silicon-carbon (Si–C) composite as an anode, to identify possible energy density improvement after pre-lithiation. These systematic formulas with great universality have the potential to give significant guidelines for future studies on the pre-lithiation methods and be useful tools for the design of high energy LIBs with imperfect CEs from fundamental and practical perspectives.

A comparison of high voltage outgassing of LiCoO2, LiNiO2, and Li2MnO3 layered Li-ion cathode materials

One method of increasing the energy density of Li-ion batteries is to access reversible Li intercalation in conventional layered transition metal oxide cathode materials at high potentials (4.3–5 V vs. Li/Li+), and thus allow more electrochemical capacity per volume of active material. This comes at the cost of increased interfacial reactivity and often results in capacity fade over many cycles. Tracing gas evolution during electrochemical lithium extraction and insertion provides a useful strategy to understand this high voltage reactivity. In this study, we examine outgassing during Li extraction in well-known layered oxides (LiCoO2, LCO; LiNiO2, LNO; and Li2MnO3, LMO). We highlight key differences in the outgassing of each material. Whereas negligible O2 release is found in LCO and LNO, even with voltage holds at 5 V vs. Li/Li+, O2 release is found to account for a large portion of the electrochemical capacity and persists over many cycles in LMO. CO2 outgassing is observed to varying extents, following the trend of LMO ≈ LNO > LCO, and is due to a combination of residual solid carbonate oxidation and electrolyte degradation. Taken together, these results show the importance of quantitative analysis in understanding the role of transition metal composition on the chemistry of conventional Li-ion battery materials.

(Invited) Superstructure Controls First Cycle Voltage Hysteresis in O-Redox Cathode Materials

The cathode is a major constraint on the energy density of Li-ion batteries. State-of-the-art materials, such as Li(Mn0.8Ni0.1Co0.1)O2, are limited in their ability to store charge by the amount of available transition metal redox. It is now known these limits can be surpassed by storing charge on the oxide ions (oxygen redox) in so-called lithium-rich transition metal (TM) oxides, e.g. Li[Li0.2Ni0.2Mn0.6]O2. However, such O-redox cathodes suffer from several problems one of which is that, in almost all known cases, the high voltage plateau associated with oxide oxidation is followed by reduction at a much lower voltage (1st cycle hysteresis) leading to a substantial drop in energy density. Here, we discuss the origin of 1st cycle voltage hysteresis and show that it can be controlled by the superstructure ordering within the TM layer.We compare two layered Na-ion O-redox compounds as model systems with very similar compositions and structure but divergent 1st cycle voltage profiles. In P2-type Na0.75[Li0.25Mn0.75]O2 with honeycomb ordering of Li and Mn in the TM layer, present in almost all O-redox compounds, substantial voltage hysteresis is observed. Whereas, for P2-type Na0.6[Li0.2Mn0.8]O2 with unique ribbon ordering, reversible high voltage O-redox is realised. The origin of voltage hysteresis is shown to be rooted in the formation of molecular O2 on charge that is trapped in in the bulk of honeycomb ordered cathodes and which can be reduced on discharge. O2 formation can be suppressed with ribbon ordering in the TM layer. The implications for future O-redox materials will also be considered. Figure 1

(Invited) Enabling Fast Charging Li-ion Batteries through Three-dimensional Graphite Anode Design

Current Li-ion battery technology is highly optimized for performance at relatively slow charging rates. However, significant challenges still present for fast-charge conditions with < 15-minute charge time.1 In state-of-the-art Li-ion batteries with high energy densities, thick electrodes (> 50 μm) are adopted, which leads to a tradeoff between energy density and high-power performance. Thicker electrodes with tortuous pathways limit Li-ion transport through the electrode thickness, resulting in large electrolyte concentration gradients during cycling. This causes large cell polarizations, which reduce the accessible capacity of the battery. In addition, the electrochemical potential of the graphite anode can become more negative than the thermodynamic potential of Li metal during fast charging, resulting in irreversible Li plating. Therefore, to simultaneously achieve fast charging and maintain energy density of Li-ion batteries, new approaches are required to address Li ionic transport limitations through the thick graphite anodes.In this work, we demonstrate a structural modification of conventional graphite anodes to improve their fast-charge capability. This is achieved by introducing laser-patterned vertical channels into post-calendered graphite anodes.2 This 3-D electrode architecture consists of a hexagonal close-packed array of vertical channels that serve as linear pathways for rapid ionic diffusion through the electrode thickness, allowing for a more homogeneous Li-ion flux throughout the volume of the electrode and decreased ionic concentration gradients. As a result, the accessible capacity of the electrode can be significantly improved and Li plating can be minimized during fast charging. Utilizing the 3-D electrode design, we demonstrate significant improvement in capacity fade at 4C (15-minute) and 6C (10-minute) charge rate with industrial-relevant electrode material and loading (> 3 mAh/cm2 electrode loading in > 2Ah pouch cells). This work thus demonstrates the viability of realizing high energy density Li-ion batteries with fast charge capability based on thick electrodes.Reference Ahmed, S. et al. Enabling fast charging e A battery technology gap assessment. 367, 250–262 (2017).Chen, K.-H; Namkoong, M.; Goel, V.; Yang, C.; Mazumder, J; Thornton, K.; Sakamoto, J; Dasgupta, N. P. Efficient Fast-Charging of Lithium-ion Batteries Enabled by Laser-Patterned Three-Dimensional Graphite Anode Architectures. J. Power Sources In Press (2020).

Porous Silicon-Cobalt Silicide-Carbon Composites for Reversible Lithium Storage

With increasing use of portable electronic devices and electric vehicles, demand for development of high-energy density rechargeable batteries is rising. To meet the market demand for long-lasting rechargeable batteries, energy density of Li-ion batteries (LIBs) should be increased. The energy density of batteries is determined by the capacities of electrodes. Carbonaceous materials have been used as negative electrodes (anodes) in commercial LIBs because of their low operating potential and reasonable cycle life. However, the theoretical capacity of graphite anodes is relatively low, which is 372 mAh g-1 for the fully intercalated LiC6 phase. To increase the capacity, different types of materials using alloying or conversion reaction should be developed. Li-alloy-based materials can store several Li ions with an atom by electrochemical alloying reactions and therefore, high capacity can be obtained. In particular, Si has a high theoretical capacity of 3580 mAh g-1 for Li15Si4 and low operating potential. However, serious capacity fading usually occurs because of huge volume changes during cycling and their low electric conductivity. To address this problem, many strategies have been developed through changes in materials design. For example, incorporation of carbon is the most famous method to solve the capacity fading of Li-alloy-based materials upon cycling. The existence of carbon in Si matrix can buffer the volume expansion of Si materials with increased electric conductivity, thus simultaneously improving both cycling and rate performance. Porous structure can be another strategy to relieve the volume expansion by using the internal pores. However, the process is generally time and energy consuming. In this study, we report a simple preparation process of porous Si-CoSi2-C composites as anode materials for LIBs. The porous composite was synthesized through cheap starting materials and cost-effective methods. Si, magnesium silicide, and cobalt oxide were mixed using high energy mechanical milling (HEMM). Then, a carbon material was incorporated by HEMM. To produce porous structure, MgO formed by HEMM was removed by a wet etching process. As a result, porous Si-CoSi2-C composites were prepared. These composites exhibited a reversible capacity of 1100 mAh g-1 and good capacity retention after 100 cycles. The porous structure and the amorphous carbon matrix alleviated the volume change to allow the enhanced electrochemical performance.

Study on Li Pre-Doping Technique with Li-Naphthalene Solutions Toward Si Negative Electrode for Next-Generation Batteries

Si is one of the most attractive negative electrode materials for balanced design of high energy density Li-ion, Li-O2 and Li-S batteries because of the high theoretical capacity of 3580 mAh g-1 delivered by Li3.75Si. Li pre-doping should be an essential technology to reduce the irreversible capacity of Si negative electrodes in the first cycle or to load Li source to positive electrode materials free of Li such as for Li-O2 or Li-S batteries.Various methods of Li pre-doping have been reported: direct contact with Li metal foil or a solution of organo-lithium radical salt like n-butyl lithium, electrochemical Li pre-doping, and mixing Li metal, etc. [1]. So far, we have reported that Si electrode pre-doped by direct contact with Li metal in an electrolyte solution containing fluoroethylene carbonate showed an excellent cycle life and high Coulombic efficiency [2]. Although this method is suitable to shallow impregnation for thin Si electrodes, it is not applicable to thick and dense electrodes of practical use [3].Therefore, we focused on method of solution of organo-lithium, Li-naphthalene, to pre-dope Li into Si. As for the method, the choice of solvent is critical to control the reactivity such as reaction velocity and penetration depth of Li. Five kinds of ether solvents were adopted in this study: three of cyclic ethers i.e. tetrahydrofuran (THF), 2-methyltetrahydrofuran (MeTHF) and tetrahydropyran (THP), and two of linear chain ethers i.e. dimethoxy ethane (G1) and trietylene glycol dimethyl ether (G3). Figure 1 shows discharge capacities of Li pre-doped Si electrodes as a function of pre-dope time at 30 °C. It is explicit that the discharge capacities of cyclic ethers such as MeTHF, THP and THF are higher than those of chain ethers such as G1 and G3. Moreover, MeTHF, THP, and G3 exhibited monotonous increasing capacities as pre-doping time increases, while THF and G1 showed decreasing tendencies. The XRD diffraction results exhibited the formation of Li3.75Si in the Si electrodes doped in the cyclic ether solutions for 24 hours. Figure 2 shows that the equilibrium potential of each Li-NTL solution decreased with increasing temperature in the order of G3 > G1 > THF > THP, MeTHF. The equilibrium potential is a good index of formation of Li-NTL radical salt. Therefore, the order of the potential is consistent with the order of discharge capacities shown in Figure 1, although the order of G1 and G3 is inconsistent between them. Figure 2 also suggests clear dependency of pre-doping rate on temperature: The higher the temperature, the higher the pre-doping rate. The dissociation degree of Li-NTL solutions were estimated from the Walden plots of the solutions by using the data of the fluidity and ionic conductivity. The result demonstrated that the dissociation degree of Li and naphthalene radical was in the order of MeTHF << THP ≤ THF. Considering the above results, this should indicate that Li-NTL(solvent)n contact ion pairs are relatively stable in MeTHF or THP solution without the reductive decomposition compared with those in THF solutions.The present results reveal that Si electrode can be effectively pre-doped with Li by using Li-NTL solution, and that the choice of solvent for Li-NTL solution is one of the critical factors to obtain high Li pre-doping level. Li3.75Si phase was formed by pre-doping for 24 hours in the Li-NTL solution using MeTHF or THP, achieving high Li pre-doping levels over 3000 mAh g-1. The solution pre-doping adopting Li-NTL is one of the promising methods for deep Li-doping, revealing that the lower equilibrium potentials resulting from deep Li doping is due to stable Li-NTL(solvent)n contact ion pairs.This work was partially supported by NEDO RISING2 project (JPNP16001).

Comparing Charge Compensation Mechanisms of Li1.3Nb0.3Mn0.4O2 and Li1.3Nb0.3Fe0.4O2 As Novel Cathode Material

Li-rich oxides have generated widespread interest for their use as next generation high energy density Li-ion battery cathodes. Among Li-rich candidates, cation disordered phases are novel lithium intercalation compounds that warrant further fundamental characterization. Here, we compare Li1.3Nb0.3Mn0.4O2 and Li1.3Nb0.3Fe0.4O2 system to explore differences in charge compensation and electrochemical performance. Using Operando XANES/EXAFS measurements examine how the Mn/Fe/Nb local environments and oxidation states change and their effect on electrochemistry. Li1.3Nb0.3Mn0.4O2 displays capacities beyond the traditional transition metal redox (Mn3+/4+), which is considered accessible through oxygen redox. Li1.3Nb0.3Fe0.4O2 exhibits charge capacities at similar potentials to that of anionic redox but our investigations suggest the charge compensation mechanism displays more Fe3+/4+ redox in origin. By using a suite of characterization techniques, we explore the capacity contributions of cation and anionic components in these systems.

Thermodynamics of Spinel-like Cation Partial Ordering in Ultrahigh Power and Energy Density Li-Ion Batteries for Fast-Charging Electric Vehicles

Recently, a class of new materials exhibiting partial spinel-like cation order, but with excess Li and excess cation content was found to combine very high rate performance with high energy density [1]. Such spinels with a cation to anion ratio higher than 3 are unusual as it requires the face sharing of some tetrahedral and octahedral cations. In this presentation we will report on the ab-initio modeling of the metastable cation arrangements in these lithium manganese oxyfluorides that are responsible for the high-rate performance. The high dimensional configurational space of the materials is described by the cluster-expansion technique and parameterized with multiple hundreds of DFT calculations.Local, atomic-scale features, such as Li-Mn dumbbell formation, antisite defects, and interstitial defects giving rise to face-sharing cations which are challenging to resolve with state-of-the-art diffraction techniques are possible to directly evaluate in our computational approach. We present a computational analysis on how cation excess in spinel-like materials is accommodated and how local cation perturbations ultimately influence short-range order and bulk Li transport. Our understanding of how features in cation-excess spinel-like Li-Mn-O-F materials influence their high rate capabilities may motivate and accelerate design of other high-performance, fast-charging battery materials.[1] Ji, H., Wu, J., Cai, Z. et al. Ultrahigh power and energy density in partially ordered lithium-ion cathode materials. Nat Energy 5, 213–221 (2020).