High Power Density Lithium-ion Batteries Research Articles

(Invited) Structured Electrode Manufacturing for Next-Generation Lithium-Ion Batteries

Achieving high-energy and high-power density Lithium-ion batteries (LIBs) with fast charging capabilities is critical to advancing electric vehicle (EV) and portable electronic technologies. Long-range, fast charge EVs and more compact portable electronics require significant advances in battery performance, lifetime, and cost. Conventional LIBs are composed of flat anode and cathode electrode stacks that can be optimized for energy or power, but not both concurrently. This is due to ion transport limitations that persist with increases in electrode thickness. Several research efforts have focused on optimizing LIB performance through material innovations with lithium metal and silicon anodes or new cathode materials, to name a few. Structured Electrode (SE) designs1 have been investigated over the last couple of decades as an alternate solution to address these challenges. SEs engineer electrode materials into different three-dimensional (3D) architectures on a scale ranging from tens to hundreds of microns to facilitate rapid ion-transport in thicker electrodes. While a promising concept, reliable and scalable manufacturing methods for fabricating SEs over the large and complex areas needed for EV and portable electronic applications remains limited.This talk covers manufacturing approaches investigated by our research group to fabricate SEs rapidly and efficiently. Both computational design and experimental approaches are employed to facilitate the development of new hardware, software, and processing techniques for SEs. Specifically, we highlight Additive Manufacturing (AM) tools for portable electronic applications and the use of acoustophoresis for manufacturing EV SEs. First, we have developed new computational fabrication tools2 and material formulations to push the bounds of AM for SE fabrication. Through this work, we investigate the impact of electrode design and fabrication on the rate capability and charge performance of Li4Ti5O12 and LiFePO4 cells. Next, taking AM towards larger area fabrication, we have combined acoustophoresis with AM principles to facilitate large-area printing and patterning of SEs for LIBs.3 Acoustophoresis utilizes acoustic standing wavesto drive particles into predefined patterns, with micron-scale control at time scales < 1 second. Our initial results with LiNi0.6Mn0.2Co0.2O2 and graphite have defined process conditions that enable acoustophoretic fabrication of SEs with improved rate capability over conventional, flat LIB electrodes. Our work highlights the material and processing considerations that must be given when developing new manufacturing processes for SEs. References C.-H. Hung, P. Huynh, K. Teo, and C.L. Cobb, “Are Three-dimensional Batteries Beneficial? Analyzing Historical Data to Elucidate Performance Advantages,” ACS Energy Letters, 8 (1), pp. 296–305, 2023.F. Fossdal, V. Nguyen, R. Heldal, C.L. Cobb, and N. Peek, “Vespidae: A Programming Framework for Developing Digital Fabrication Workflows,” In Proc. of 2023 Designing Interactive Systems Conference, ACM, New York, NY, USA.K.E. Johnson, D.S. Melchert, E.N. Armstrong, Daniel S. Gianola, C.L. Cobb, M.R. Begley, “A Simple, Validated Approach for Design of Two-dimensional Periodic Particle Patterns via Acoustophoresis,” Materials & Design, Vol. 232, 112165, 2023. Acknowledgements This material is based upon work supported in part by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Materials and Manufacturing Technologies Office (AMMTO) Award Numbers DE-EE0009112 and DE-EE0010226. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.This work was funded in part by a Defense Advanced Research Projects Agency (DARPA) Young Faculty Award and Director's Fellowship under grant number D19AP00038. The views, opinions, and findings expressed in this work are those of the authors and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government, and no official endorsement should be inferred. This is approved for public release and distribution is unlimited.

The Facile and Controllable Synthesis of Ultrafine Sn Nanocrystals Loaded on Carbon Black for High-Performance Lithium Storage.

Sn and C nanocomposites are ideal anode materials for high-energy and high-power density lithium ion batteries. However, their facile and controllable synthesis for practical applications is still a critical challenge. In this work, a facile one-step method is developed to controllably synthesize ultrafine Sn nanocrystals (< 5 nm) loaded on carbon black (Sn@C) through Na reducing SnCl4 by mechanical milling. Different from traditional up-down mechanical milling method, this method utilizes mechanical milling to trigger bottom-up reduction reaction of SnCl4. The in-situ formed Sn nanocrystals directly grow on carbon black, which results in the homogeneous composite and the size control of Sn nanocrystals. The obtained Sn@C electrode is revealed to possesses large lithium diffusion coefficient, low lithiation energy barrier and stable electrochemical property during cycle, thus showing excellent lithium storage performance with a high reversible capacity (942 mAh g-1 at a current density of 100 mA g-1), distinguished rate ability (480 mAh g-1 at 8000 mA g-1) and superb cycling performance (730 mAh g-1 at 1000 mA g-1 even after 1000 cycles).

Borate modified Co-free LiNi0.5Mn1.5O4 cathode material: A pathway to superior interface and cycling stability in LNMO/graphite full-cells

Cobalt-free LiNi0.5Mn1.5O4 (LNMO) holds great promise as a next-generation cathode material for high-energy and high-power density lithium-ion batteries (LIBs), making it a key contender for large-scale energy storage systems (ESS) and transportation applications. Despite its potential, the commercial utilization of LNMO has been impeded by its rapid capacity deterioration attributed to an unstable LNMO/electrolyte interface caused by the intrinsically high operating voltage of LNMO. Here, we present a pragmatic and scalable strategy to improve the LNMO/electrolyte interface through borate-based surface coating of the LNMO. Optimizing the coating process yielded high-rate capability and stable LNMO/electrolyte interface analyzed by extended float tests. Borate-LNMO materials show superior cycling stability at 25 °C and 45 °C, attributed to a robust cathode electrolyte interphase (CEI) formation. The bare LNMO and 0.25-B2O3-LNMO demostrated a cycling stability of around 41.20 % and 62.50 % respectively after 1000 cycles in LNMO/graphite full-cells. Post-mortem X-ray photoelectron spectroscopy (XPS) analysis revealed fewer side reaction products on borate-LNMO cathodes compared to bare LNMO. Additionally, scanning electron microscopy (SEM) revealed a thicker and denser solid-electrolyte interphase (SEI) on cycled graphite anodes from bare LNMO cells, while a thin and robust SEI was observed on graphite from borate-LNMO cells. Finally, we have conclusively demonstrated that the primary aging mechanism in LNMO/graphite full cells stems from the instability at the LNMO/electrolyte or/and graphite/electrolyte interface, rather than material-related degradation phenomenon. These compelling outcomes underscore the potential of cathode material surface coating in general and especially of borate coating on LNMO as a promising and cost-effective enabler for the use of LNMO in next-generation LIBs.

Data-physics-driven estimation of battery state of charge and capacity

High-power density lithium-ion batteries have been utilized in both energy storage and high rate charging and discharging applications. Accurate state estimation is fundamental to enhancing battery life and safety. Therefore, a data-physics-driven estimation of the state of charge and capacity for lithium-titanate batteries was conducted using Gaussian distribution fusion. Firstly, a fractional order model was selected as the physical analytical model for lithium-titanate batteries. Secondly, a data-driven model that combines convolutional neural networks and long short-term memory networks was employed to predict the battery state of charge. Thirdly, the physical analytical model and data-driven model were fused to estimate the state of charge by employing the principle of Gaussian distribution fusion. Finally, both the state of charge and actual capacity of the battery were jointly estimated. The results showcase the capability of the proposed method to accurately estimate the state of charge and capacity, ensuring an accuracy level within 1%. Furthermore, this approach exhibits a 20% improvement in accuracy compared to traditional methods.

Borate-Coated Co-Free LiNi0.5Mn1.5O4: Enhanced Performance and Stability for High-Power-Density Libs

Cobalt free LiNi0.5Mn1.5O4 (LNMO), also known as high-voltage spinel, has emerged as a promising cathode material for high energy-density and high power-density lithium-ion batteries (LIBs), making it a viable candidate for applications in large-scale energy storage systems (ESS) and transportation (1-5). Despite its potential, LNMO faces challenges, such as rapid capacity degradation and the formation of an unstable cathode electrolyte interphase (CEI), which have impeded its commercialization(4-5). To address these issues, we present a cost-effective and scalable approach involving the application of a borate-based surface coating to LNMO (borate-LNMO). In this study, we systematically applied varying amounts of borate coating to LNMO and assessed their electrochemical performance in both half- and full-cell configurations. Initial optimization of the coating amount revealed that borate-LNMO materials exhibited superior rate capability, and enhanced stability when compared to bare LNMO. Float testing demonstrated a stable LNMO/electrolyte interface for borate-LNMO materials, in contrast to the continuous increase in leakage (parasitic current) observed with bare LNMO over time. Furthermore, borate-LNMO materials exhibited superior cycling performance in full-cell setups, both at ambient (25℃) and elevated (45℃) temperatures. This enhanced performance can be attributed to the formation of relatively stable CEI and SEI interphases, resulting in reduced electrolyte decomposition, lower transition metal dissolution at the LNMO/electrolyte interface, and minimized cross-talk between the cathode and anode. Post-mortem SEM analysis of the cycled graphite anodes revealed a thicker and denser SEI in bare LNMO cells, whereas borate-LNMO cells exhibited a thinner and porous SEI. These findings suggest that borate-coated LNMO could be a promising solution for cost-effective and high-power LIBs.

(Invited) Manufacturing 3D Electrode Architectures Via Acoustophoresis

Achieving high-energy and high-power density Lithium-ion batteries (LIBs) with fast charge behavior is critical for the future of electric vehicle applications. Conventional LIBs have planar anode and cathode electrode stacks that can be optimized for energy or power, but not both simultaneously due to fundamental ion transport limitations with increasing electrode thickness. Three-dimensional (3D) electrode architectures1,2 can remove these performance trade-offs through engineered ion-transport in thick electrodes. However, scalable manufacturing methods for patterning these architectures over large areas at meaningful time scales is still limited. As a path to solving this challenge, we leverage both modeling and experiments to investigate the feasibility of deploying acoustophoresis to assemble and pattern 3D battery electrodes. Acoustophoresis employs acoustic standing waves to focus particles and enables near micron-scale control over particle placement in a fluid medium at time scales < 1 second. Prior research has shown the potential for rapid assembly of particles with this approach,3,4 making acoustic-based processing a promising methodology for manufacturing 3D electrodes over large areas. In this talk, we expand a previously developed model4 that solves differential equations of acoustic forces to track particle trajectories and define how the acoustic forces are influenced by slurry viscosity, particle loading, and particle morphology. Our initial experiments with different material systems, including LiNi0.6Mn0.2Co0.2O2 (NMC-622), help validate our model and process conditions to as a path towards acoustophoretic fabrication of 3D electrode architectures. References C. L. Cobb and S. E. Solberg, J. Electrochem. Soc., 164, A1339–A1341 (2017).C. L. Cobb and M. Blanco, Journal of Power Sources, 249, 357–366 (2014).D. S. Melchert et al., Materials & Design, 109512 (2021).R. R. Collino et al., Materials Research Letters, 6, 191–198 (2018). Acknowledgements This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Manufacturing Office (AMO) Award Number DE-EE0009112. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Selective lithiation and lithium induced nano sticks formation unveil caffeine for ultra-long-term stability at high C-rate and high power density lithium-ion battery

Molecular organic material is of great interest for lithium ion battery applications due to its abundance, tunable redox properties, and harmless disposal. 1,3,7-trimethylxanthine (Caffeine) is a naturally occurring alkaloid, having multiple redox-active centres such as carbonyls (C = O), alkene (C = C) and the imine (C = N) for lithium-ion doping. The electrochemical reaction with lithium-ion results in a highly reversible capacity of 313 mAh g−1 at 1 C and remains stable for 600 cycles. The electrochemical storage of lithium ions in caffeine anode was revealed by selective doping at C = N and C = O located in between the tertiary-nitrogen. Caffeine anodes are ultra-stable operated at 10 C, up to 6000 cycles. Caffeine's extraordinary stability is attributed to dope-induced transformation of micro-rods into nano sticks. The caffeine-LiCoO2 full battery system delivers a voltage of 3.95 V with a stable specific capacity and promising for practical applications.

Hollow multi-nanochannel carbon nanofiber/MoS2 nanoflower composites as binder-free lithium-ion battery anodes with high capacity and ultralong-cycle life at large current density

The electrode materials with high pseudocapacitance can enhance the rate capability and cycling stability of lithium-ion storage devices. Herein, we fabricated MoS2 nanoflowers with ultra-large interlayer spacing on N-doped hollow multi- nanochannel carbon nanofibers (F2-MoS2/NHMCFs) as freestanding binder-free anodes for lithium-ion batteries (LIBs). The ultra-large interlayer spacing (0.78∼1.11 nm) of MoS2 nanoflowers can not only reduce the internal resistance, but also increase accessible active surface area, which ensures the fast Li+ intercalation and deintercalation. The NHMCFs with hollow and multi-nanochannel structure can accommodate the large internal strain and volume change during lithiation/delithiation process, it is beneficial to improving the cycling stability of LIBs. Benefiting from the above combined structure merits, the F2-MoS2/NHMCFs electrodes deliver a high rate capability 832 mA h g−1 at 10 A g−1 and ultralong cycling stability with 99.29 and 91.60 % capacity retention at 10 A g−1 after 1000 and 2000 cycles, respectively. It is one of the largest capacities and best cycling stability at 10 A g−1 ever reported to date, indicating the freestanding F2-MoS2/NHMCFs electrodes have potential applications in high power density LIBs.

Supercharged!

Supercharged!

Novel composite separator for high power density lithium-ion battery

High power density is essential for next-generation lithium-ion batteries. However, it is a great challenge to achieve high-power density and safety lithium-ion batteries due to the lack of reliable separators with uniform porosity, strong mechanical flexibility, and superior electrolyte uptake. Herein, a poly (vinylidene fluoride-co-hexafluoropropylene)/graphene oxide separator with uniform porosity, strong mechanical flexibility, and superior electrolyte uptake is prepared. The incorporation of graphene oxide into a poly (vinylidene fluoride-co-hexafluoropropylene) separator provides a large number of uniform diffusion channels for lithium ions to pass through by reducing the degree of crystallinity and increasing the pore uniformity of separator. In addition, the separator has superior mechanical property and thermal stability in comparison to commercial Celgard 2325 and pure poly (vinylidene fluoride-co-hexafluoropropylene) separators. This LiFePO4/Li cell with the prepared poly (vinylidene fluoride-co-hexafluoropropylene)/graphene oxide separator delivers a high capacity up to 160 mAh g−1 at 0.5C at room temperature and can be fully charged/discharged in 6 min (10C).

Polyphenylene Sulfide Separator for High Safety Lithium-Ion Batteries

Separators with ultra-high strength, superior heat-resistance and excellent electrolyte wettability are in urgent demand for the development of high safety and high-power density lithium-ion batteries. Herein in this paper, specially designed polyphenylene sulfide separator is prepared via thermally induced phase separation using binary diluent to improve the performance of LIBs for the first time. The effect of binary diluent composition on the polyphenylene sulfide separator performance is investigated by both computational simulation and experimental studies respectively. The as-prepared polyphenylene sulfide separator exhibits tortuous and highly porous structure, which endows the separator superior electrolyte wettability. Furthermore, it also results in higher ionic conductivity (1.69 mS cm−1), lithium-ion transference number (0.57) and interfacial stability of the polyphenylene sulfide separator. In particular, the polyphenylene sulfide separator presents excellent thermal dimensional stability (without shrinkage up to 280°C), self-extinguishing and ultra-high strength (tensile strength of 119 MPa, Young's modulus of 7.55 GPa) benefiting from the high performance polyphenylene sulfide. More importantly, the LiFePO4/Li cell using the polyphenylene sulfide separator shows satisfactory cycling stability (0.064% capacity fading per cycle at 2 C) and superior rate performance. These desirable performances suggest that the polyphenylene sulfide separator is very attractive for high safety and high-power density LIBs.

Cu-Substituted NiF2 as a Cathode Material for Li-Ion Batteries.

Metal fluorides usually have a large electronegativity and are promising electrode materials for high-power lithium-ion batteries. However, like other conversion-reaction-based materials, large volumetric expansions and large capacity losses in cycling are the major issues for metal fluorides. Here, we explore substitution of Ni with Cu for binary NiF2 and its effects on the electrochemical properties. By in situ transmission electron microscopy, the structural evolutions of several ternary metal fluorides with different Cu/Ni ratios are observed and correlated with their electrochemical properties. With increased Cu substitution from 0 to 25 wt %, the areal expansion during the first lithiation is reduced. Meanwhile, the fluorine loss (due to reaction irreversibility) in the delithiation cycle is also reduced. This provides an explanation for the advantage of Cu substitution for improved cycling stability and capacity. We believe that our observations provide insight into the development of better ternary metal fluorides as cathodes for high power density lithium-ion batteries.

Toward Low-Cost, High-Energy Density, and High-Power Density Lithium-Ion Batteries

Reducing cost and increasing energy density are two barriers for widespread application of lithium-ion batteries in electric vehicles. Although the cost of electric vehicle batteries has been reduced by ~70% from 2008 to 2015, the current battery pack cost ($268/kWh in 2015) is still >2 times what the USABC targets ($125/kWh). Even though many advancements in cell chemistry have been realized since the lithium-ion battery was first commercialized in 1991, few major breakthroughs have occurred in the past decade. Therefore, future cost reduction will rely on cell manufacturing and broader market acceptance. This article discusses three major aspects for cost reduction: (1) quality control to minimize scrap rate in cell manufacturing; (2) novel electrode processing and engineering to reduce processing cost and increase energy density and throughputs; and (3) material development and optimization for lithium-ion batteries with high-energy density. Insights on increasing energy and power densities of lithium-ion batteries are also addressed.

Developing Li-Metal Alloy Batteries with High Cyclability and Rate Capability

The development of high-energy density and high-power density lithium ion batteries (LIBs) for portable electronics, electric vehicles, and other large-scale energy storage devices over the last decade has led to continual search on new electrode candidates. The cost-effective, chemically stable (safe), and highly conductive metal as anode electrode that is capable of high capacity due to the large Li stoichiometric ratio, has recently stimulated enormous attention. There have been many efforts to increase the capacity using group IV elements Sn, Ge, Pb, Si and Mg, Al, Ga, Zn, and Bi thus far. Sn, another group IV element, is a promising alternative anode material, as its bulk phase is known to have a higher theoretical capacity of 990 mAh/g (Li4.4Sn) compared to 370 mAh/g of commercial graphite. Sn is also less toxic and less expensive, but huge volume changes up to 300% during Li insertion and extraction limit the energy capacity and cycle lifetime in LIBs. In addition to the volume factor, another obstacle lies in the naturally formed oxide layer, which introduces a non-reversible capacity, although some researchers have found that an ultrathin coating on electrodes could enhance the performance of lithium-ion batteries. To maximize the electrochemical performance of Sn, a well-desired configuration that can efficiently prevent the chemo-mechanical failure must be developed. Here we present a novel sub-100nm Sn nano-cage (Sn-NC) comprising an industrially available, pure metal Sn and a conductive oxide surface (not SnOx). The protection of the rigid oxide surface and the adjustable interspace can prevent the pulverization and oxidation of the metal while supplying a fast Li+/e- pathway. The coin/punch cell assembled using Sn-NCs as anode provides a high capacity (>600 mAh/g) with a long cyclability (>500 cycles) at a high charge/discharge rate. The synthesis of Sn-NCs is scalable thus indicating a potential on industrial commercialization.

Poly(m-phenylene isophthalamide) separator for improving the heat resistance and power density of lithium-ion batteries

A microporous poly(m-phenylene isophthalamide) (PMIA) separator with high safety (high-heat resistance and self extinguishing), high porosity and excellent liquid electrolyte wettability was prepared by the traditional nonsolvent introduced phase separation process. Due to the high-heat resistance of PMIA material, the as-prepared separator exhibited a negligible thermal shrank ratio at 160 °C for 1 h. Meanwhile, benefiting from its high porosity and excellent wettability in liquid electrolyte, the liquid electrolyte uptake and the ionic conductivity of the separator were higher than that of the commercial PP-based separators. Furthermore, the cell assembled with this separator showed better cycling performance and superior rate capacity compared to those with PP-based separators. These results suggested that the PMIA separator is very attractive for high-heat resistance and high-power density lithium-ion batteries.

Enhanced Rate Capability in Two Phase Reaction in LiFePO4 By Interfacial Modification Between Cathode and Electrolyte

Olivine-type LiFePO4 is considered as a promising cathode material for high power density lithium-ion battery. Lithium-ion intercalation proceeds thorough a two-phase reaction divided into LiFePO4 and FePO4. During the phase transition between LiFePO4 and FePO4, the nucleation of a new phase occurs at the electrode / electrolyte interface, which is reported to be the rate determining reaction.1 To improve the rate capability of LiFePO4, Park et al performed the surface modification by nitrogen on LiFePO4 particles.2 However, their improved mechanism is not clearly understood due to the difficulty in the observation of electrode / electrolyte interface. In order to analyze the electrode / electrolyte interface, thin-film electrodes and surface sensitive X-ray absorption spectroscopy are powerful tool.3 In this study, we prepared LiFePO4thin-film model electrodes to clarify the reaction kinetics and applied the interfacial modification of nitrogen. Their reaction mechanism is discussed with the information of X-ray absorption spectroscopy. LiFePO4 thin-film was prepared on a polycrystalline Au substrate by pulsed laser deposition method. The deposition was conducted under Ar atmosphere (0.0001 Pa) at 923K for 30 min. The prepared thin films were annealed under ammonia gas at 873 K. The obtained thin-film was characterized by X-ray diffraction and TEM. X-ray absorption spectroscopy at Co K-edge was performed at BL37XU, Spring-8. N K-edge X-ray absorption measurements were performed in Ritsumeikan SR-center. For the electrochemical analysis, three-electrode cells employing Li metal as a counter and a reference electrode, and 1 M LiClO4in propylene carbonate liquid electrolyte were prepared. Annealing of ammonia gas introduces nitrogen into LiFePO4 surface, which is confirmed by N K-edge X-ray absorption spectra. The rate capability of the nitrogen modified LiFePO4 thin-film electrodes is improved. The surface sensitive Fe K-edge X-ray absorption spectra show the increasing of oxidation state in iron, which implies the lattice mismatch is decreased in surface of LiFePO4. Therefore, nucleation reaction is enhanced during two phase reaction between LiFePO4 and FePO4, which is the origin of surface modification effect.

Microwave exfoliated graphene oxide/TiO2 nanowire hybrid for high performance lithium ion battery

Lithium ion battery (LIB) is a key solution to the demand of ever-improving, high energy density, clean-alternative energy systems. In LIB, graphite is the most commonly used anode material; however, lithium-ion intercalation in graphite is limited, hindering the battery charge rate and capacity. To date, one of the approaches in LIB performance improvement is by using porous carbon (PC) to replace graphite as anode material. PC's pore structure facilitates ion transport and has been proven to be an excellent anode material candidate in high power density LIBs. In addition, to overcome the limited lithium-ion intercalation obstacle, nanostructured anode assembly has been extensively studied to increase the lithium-ion diffusion rate. Among these approaches, high specific surface area metal oxide nanowires connecting nanostructured carbon materials accumulation have shown promising results for enhanced lithium-ion intercalation. Herein, we demonstrate a hydrothermal approach of growing TiO2 nanowires (TON) on microwave exfoliated graphene oxide (MEGO) to further improve LIB performance over PC. This MEGO-TON hybrid not only uses the high surface area of MEGO but also increases the specific surface area for electrode–electrolyte interaction. Therefore, this new nanowire/MEGO hybrid anode material enhances both the specific capacity and charge–discharge rate. Scanning electron microscopy and X-ray diffraction were used for materials characterization. Battery analyzer was used for measuring the electrical performance of the battery. The testing results have shown that MEGO-TON hybrid provides up to 80% increment of specific capacity compared to PC anode.

Research Progress in Improving the Rate Performance of LiFePO4 Cathode Materials

Abstract Olivine lithium iron phosphate (LiFePO4) is considered as a promising cathode material for high power-density lithium ion battery due to its high capacity, long cycle life, environmental friendly, low cost, and safety consideration. The theoretical capacity of LiFePO4 based on one electron reaction is 170 mAh g−1 at the stable voltage plateau of 3.5 V vs. Li/Li+. However, the instinct drawbacks of olivine structure induce a poor rate performance, resulting from the low lithium ion diffusion rate and low electronic conductivity. In this review, we summarize the methods for enhancing the rate performance of LiFePO4 cathode materials, including carbon coating, elements doping, preparation of nanosized materials, porous materials and composites, etc. Meanwhile, the advantages and disadvantages of above methods are also discussed.

Research Progress in Improving the Rate Performance of LiFePO4 Cathode Materials

Olivine lithium iron phosphate (LiFePO4) is considered as a promising cathode material for high power-density lithium ion battery due to its high capacity, long cycle life, environmental friendly, low cost, and safety consideration. The theoretical capacity of LiFePO4 based on one electron reaction is 170 mAh g−1 at the stable voltage plateau of 3.5 V vs. Li/Li+. However, the instinct drawbacks of olivine structure induce a poor rate performance, resulting from the low lithium ion diffusion rate and low electronic conductivity. In this review, we summarize the methods for enhancing the rate performance of LiFePO4 cathode materials, including carbon coating, elements doping, preparation of nanosized materials, porous materials and composites, etc. Meanwhile, the advantages and disadvantages of above methods are also discussed.

High-Voltage Electrolytes Based on Nitrile Containing Disiloxanes for Lithium-Ion Batteries

Two nitrile containing disiloxanes were investigated as non-volatile solvent for electrochemically stable electrolytes suitable for high energy and high power density lithium-ion batteries. The investigated disiloxanes show an excellent thermal behaviour (bp = 106 °C, 0.1 mbar) with a very low glass transition temperature T gdown to -107 °C.Fig. 1 a) CV of TmdSx-PN and b) Arrhenius plot of nitrile containing disiloxane TmdSx-OPN with different LiTFSI salt concentrations.Electrochemical characterization was performed by temperature dependent impedance spectroscopy, differential scanning calorimetry, cyclic voltammetry and constant current cycling. The siloxane electrolytes showed a wide electrochemical window up to 5.4 V vs. Li/Li+ using LiTFSI as lithium salt. High ionic conductivity was found up to 1 mS∙cm-1 at 30 °C and a molality of 0.7 mol/Kg (CN:Li = 11:1).[1] These are the highest reported ionic conductivities and anodic stability for a liquid siloxane electrolyte.[2] Charge-discharge half-cell test were done with LiFePO4cathode materials. These results show that nitrile functionalized disiloxanes are very promising electrolyte solvents for the use in lithium-ion batteries with high energy densities. In addition, nitrile functionalized disiloxanes are very suitable as liquid solvent in polymer based gel electrolytes.The authors acknowledge funding by the German federal ministry of education and research (BMBF) within the project “MEET Hi-EnD”.[1] B. Pohl, M.M. Hiller, S.M. Seidel, M. Grünebaum, H.-D. Wiemhöfer, submitted, (2014).[2] S. Jeschke, A.-C. Gentschev, H.-D. Wiemhöfer, Chem. Commun., 49 (2013) 1190-1192.