Lithium Diffusion Barrier Research Articles

Phase Evolution of Multi-Metal Dichalcogenides With Conversion-Alloying Hybrid Mechanism for Superior Lithium Storage.

Traditional lithium-ion battery (LIB) anodes, whether intercalation-type like graphite or alloying-type like silicon, employing a single lithium storage mechanism, are often limited by modest capacity or substantial volume changes. Here, the kesterite multi-metal dichalcogenide (CZTSSe) is introduced as an anode material that harnesses a conversion-alloying hybrid lithium storage mechanism. Results unveil that during the charge-discharge processes, the CZTSSe undergoes a comprehensive phase evolution, transitioning from kesterite structure to multiple dominant phases of sulfides, selenides, metals, and alloys. The involvement of multi-components facilitates electron transport and mitigates swelling stress; meanwhile, it results in formation of abundant defects and heterojunctions, allowing for increased lithium storage active sites and reduced lithium diffusion barrier. The CZTSSe delivers a high specific capacity of up to 2266mA h g-1 at 0.1 A g-1; while, maintaining a stable output of 116mA h g-1 after 10000 cycles at 20 A g-1. It also demonstrates remarkable low-temperature performance, retaining 987mA h g-1 even after 600 cycles at -40°C. When employed in full cells, a high specific energy of 562Wh kg-1 is achieved, rivalling many state-of-the-art LIBs. This research offers valuable insights into the design of LIB electrodes leveraging multiple lithium storage mechanisms.

Engineering of Cerium Modified TiNb2O7 Nanoparticles For Low-Temperature Lithium-Ion Battery.

Although TiNb2O7 (TNO) with comparable operating potential and ideal theoretical capacity is considered to be the most ideal replacement for negative Li4Ti5O12 (LTO), the low ionic and electronic conductivity still limit its practical application as satisfactory anode for lithium-ion batteries (LIBs) with high-power density. Herein, TNO nanoparticles modified by Cerium (Ce) with outstanding electrochemical performance are synthesized. The successful introduction of Ce3+ in the lattice leads to increased interplanar spacing, refined grain size, more oxygen vacancy, and a smaller lithium diffusion barrier, which are conducive to improve conductivity of both Li+ and electrons. As a result, the modified TNO reaches high reversible capacity of 256.0mAhg-1 at 100mAg-1 after 100cycles, and 183.0mAhg-1 even under 3200mAg-1. In particular, when the temperature drops to -20°C, the cell undergoing 1500cycles at a high current density of 500mAg-1 can still reach 89.7mAhg-1, corresponding to a capacity decay rate per cycle of only 0.033%. This work provides a new way to improve the electrochemical properties of alternative anodes for LIBs at extreme temperature.

Electrochemical Lithium Deposition on LixTi5O12.

Lithium metal batteries (LMBs) have been regarded as one of the most promising next-generation high-energy-density storage devices. However, uncontrolled lithium dendrite growth leads to low Coulombic efficiencies and severe safety issues, hindering the commercialization of LMBs. Reducing the diffusion barrier of lithium is beneficial for uniform lithium deposition. Herein, a composite is constructed with Li4Ti5O12 as the skeleton of metallic lithium (Li@LixTi5O12) because Li4Ti5O12 is a "zero-strain" material and exhibits a low lithium diffusion barrier. It was found that the symmetric cells of Li@LixTi5O12 can stably cycle for over 400 h at 1 mA cm-2 in the carbonate electrolyte, significantly exceeding the usual lifespan (∼170 h) of the symmetric cell using a lithium metal electrode. In a full cell with Li@LixTi5O12 as the anode, the cathode LiFePO4 delivers a capacity retention of 78.2% after 550 cycles at 3.6C rate and an N/P ratio = 8.0. This study provides new insights for designing a practical lithium anode.

Rotation angle dependent Li intercalation and the induced phase transition in bilayer MoTe2

In this work, taking twisted bilayer MoTe2 (tb-MoTe2) as an example, density functional theory (DFT) calculations are done to investigate the interlayer rotation angle dependent lithium intercalation as well as the induced 2H → 1T phase transition in two-dimensional layered materials. Both static equipotential energy diagrams and dynamic ab initio molecular dynamics simulations (AIMD) are utilized to obtain the reliable lithium diffusion pathways and barriers. The results reveal that the interlayer rotation leads to increased interlayer separation in tb-MoTe2, and provides a stable environment for Li insertion. Moreover, the diffusion barrier is substantially reduced with increasing twist angle, promoting the fast Li diffusion. At room temperature, the mobility of Li in tb-MoTe2 with the twist angle of θ = 21.79˚ and θ = 60˚ is increased by a factor of 10 and 105, respectively. The energy barrier for the 2H → 1T phase transition is lowered from 1.23 eV in the non-twisted MoTe2 to 0.88 eV in the tb-MoTe2 with a twist angle θ = 21.79˚ for a given concentration of inserted Li. Above all, the enhanced diffusion and lower critical electron concentration facilitate the 2H-1T phase transition in tb-MoTe2. The results provide valuable insights into the non-uniform intercalation and present an opportunity to modulate the phase transition of twisted 2D systems.

A theoretical investigation on the two-dimensional Fe/Mn tricarbides (XC3) as promising electrode materials for lithium-ion batteries

2D materials have shown great potential for application in metal-ion batteries due to their promising ionic and electronic conductivity as well as large capacities. In this work, we explore transition metal tricarbides as electrode material to lithium-ion batteries (LIBs) by employing CALYPSO method to conduct a structure search and then perform the first-principles density functional theory (DFT) calculation to study their electronic structure, thermodynamic stability, lithium capacity, lithium diffusion barrier and open circuit voltage. A series of two-dimensional transition metal tricarbides XC3 (X = Fe, Mn) monolayer was obtained from the structure search and four of them (marked by m-XC3 and d-XC3, X = Fe, Mn) were found to possess good dynamical, mechanical and thermodynamic stability from DFT calculation. Further calculation of electronic structure revealed a metallic character for the m-XC3 and d-XC3 (X = Fe, Mn), which ensures good electronic conductivity as electrodes of LIBs. The predicted migration energy barriers of Li diffusion on d-FeC3, d-MnC3, m-FeC3 and m-MnC3 were 0.16 eV, 0.16 eV, 0.19 eV and 0.17 eV respectively, which ensures fast charging/discharging rate. Moreover, d-FeC3, d-MnC3, m-FeC3 and m-MnC3 monolayer were found to have low open-circuit voltage of 1.511 V, 1.423 V, 0.913 V and 0.728 V, which make them suitable for the anode material in LIBs. Finally, the FeC3 and MnC3 monolayer showed theoretical specific capacity of 874 mAhg−1 and 885 mAhg−1. The predicted superior performance of Li storage and transport make the XC3 (X = Fe, Mn) monolayers to be promising candidates as the anode material of LIBs.

Theoretical prediction of two-dimensional Cr3C2 monolayer and its derivatives as potential electrode of Li-ion batteries

The investigations on Cr3C2 and Cr3C2T2 (T = F, Cl, O, OH) monolayers as promising electrode materials for lithium-ion battery (LIBs) are systematically carried out by Hubbard-corrected density functional theory (DFT + U). It indicates that Cr3C2 monolayer and its derivatives are dynamically and thermally stable. At the same time, the pristine Cr3C2 monolayer exhibits metallic character and the conductivity can be maintained through chemical modification. The lithium diffusion barrier on Cr3C2 monolayer is only 0.024 eV, which can guarantee a fast rate during the charge/discharge process. The storage capacity of Cr3C2 monolayer adsorbing one layer of Li atoms is about 298 mAh/g. It is found that the Cr3C2T2 (T = Cl, O, OH) monolayer can keep at least one layer of Li atoms adsorbed without any structural deformation, and the theoretical capacities are 214, 253 and 250 mAh/g, respectively. The Li storage capacities can be increased exponentially with the more layers of Li atoms adsorbing on the two-dimensional (2D) materials. Thus, Cr3C2 and Cr3C2T2 monolayers are potential electrode materials for LIBs.

LC567: a new 2D semimetallic carbon allotrope as a promising anode material for lithium-ion batteries.

A novel two-dimensional carbon allotrope has been proposed using density functional theory (DFT) calculations. The cell contains 24 carbon atoms and is composed of five-, six-, and seven-membered rings, named LC567. It is low in energy and has excellent dynamic, thermal, and mechanical stability. Our results demonstrate that the theoretical capacity of monolayer LC567 is up to 1117 mA h g-1, and the lithium diffusion barrier is also very low, around 0.18 eV, which is superior to graphene and most reported two-dimensional anode materials. In addition, LC567 exhibits quite low open circuit voltage during the process of Li ion insertion. For the bulk of LC567, it still exhibits high capacity and ideal open circuit voltage, revealing its potential application as an anode for lithium batteries. Meanwhile, we discuss the mechanism of the high capacity and low diffusion barrier of LC567 as an anode material for lithium batteries, and find that the high capacity and low diffusion barrier properties may be related to the pentagonal carbon rings (C5).

First‐Principles Exploration of 2D Benzenehexathiolate Coordination Nanosheets for Broadband Electrochromic Devices

AbstractElectrochromic materials can tune the illumination and heat exchange of a building with the environment and thereby save energy in lighting, heating, and air conditioning in a cost‐effective way, which is vital in realizing carbon neutrality. 2D frameworks such as coordination nanosheets (CONASHs) that are widely explored for a wide range of applications in energy storage and conversion can be a cluster of novel electrochromic materials. In this work, a series of transition metal benzenehexathiol (TM‐BHT) CONASHs are theoretically investigated via first‐principles simulations. During ion intercalation and deintercalation in TM‐BHTs, changes in lattice structures, lithium diffusion barriers, atomic charges, bond strength, and electronic properties are explored in‐depth. The incurred changes are then correlated with critical electrochromic properties, including the transmittance adjustment ranges in the visible light, near‐infrared, solar spectrum, and mid‐infrared. Among the various TM‐BHT systems, Cu‐BHT and Ag‐BHT are the most promising broadband electrochromic materials for optical and thermal management in the wavelength range from visible to mid‐infrared. The theoretical guidance from this work paves a new path toward electrochromic applications of CONASHs that exploit the versatility of these 2D materials.

Mechanochemistry-induced biaxial compressive strain engineering in MXenes for boosting lithium storage kinetics

Strain modulation can endow electrochemical materials with promising mechano-electrochemical coupling owing to its adjustable characteristics, which will unlock great potential for realizing high-performance energy storage. MXenes offer outstanding lithium storage performances due to their exceptional conductivity, excellent mechanical properties, and large interlayer spaces for ion intercalation, however an undesirable issue of sluggish kinetics caused by the restacking of MXene nanosheets in electrodes is not well addressed. Here, we demonstrate an extremely effective strategy to resolve this issue by creating strain in Ti 3 C 2 T x MXene via mechanochemistry (MC) method to maximize ion-transfer kinetics for lithium-ion batteries (LIBs). Strain states in Ti 3 C 2 T x MXene, namely out-of-plane tension and corresponding in-plane biaxial compression, are comprehensively assessed through X-ray diffraction, Raman spectroscopy, and extended X-ray absorption fine structure spectroscopy. Diverse experimental characterizations and density functional theory calculations both reveal that the mechanochemistry-induced strained Ti 3 C 2 T x (MC-Ti 3 C 2 T x ) MXene exhibits significantly decreased lithium diffusion barrier, which correlates directly to the observed fast ion-transfer kinetics. As expected, MC-Ti 3 C 2 T x electrode delivers a high discharge capacity (380.5 mAh g −1 at 0.1 A g −1 ) and superior rate capability, outperforming most of previously reported Ti 3 C 2 T x . More importantly, with the remarkably enhanced ion-transfer kinetics, MC-Ti 3 C 2 T x electrode exhibits outstanding lithium storage performances spanning a wide temperature range (40 °C to − 20 °C). This work paves a novel way of strain engineering of MXenes for effectively enhancing diffusion kinetics in LIBs. Guided by the exponential relationship between ion diffusivity and strain, we designed strained Ti 3 C 2 T x via a facile mechanochemistry strategy that features enhanced ion-transfer kinetics in Li-ion batteries. Benefitting from its out-of-plane tension and corresponding in-plane biaxial compression, the strained Ti 3 C 2 T x anode demonstrates a capacity enhancement of ~100%, significantly increased pseudocapacitive storage, and excellent temperature tolerance, as compared to the unstrained counterpart. • We designed strained Ti 3 C 2 T x MXene via an effective mechanochemistry strategy. • The strained Ti 3 C 2 T x exhibits significantly improved ion-transfer kinetics. • The fast ion-transfer kinetics promotes intercalation pseudocapacitive behavior. • The strained Ti 3 C 2 T x shows excellent lithium storage in wide temperature range.

Exploring the anchoring effect and catalytic mechanism of 3d transition metal phthalocyanine for S8/LiPSs: A density functional theory study

The strong anchoring effects for lithium polysulfides (LiPSs), as well as fast redox kinetics, are of great significance and necessity for the commercial development of lithium-sulfur batteries (LSBs). Metal phthalocyanines (MPc), with special M-N4 moieties, are a class of macrocyclic compounds that have the potential to be employed as sulfur host materials in LSBs. Herein, a series of 3d transition metal phthalocyanines (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) were systematically investigated for the anchoring effects and catalytic conversion activities for S8/LiPSs. The binding energy analysis demonstrates that most of MPc (except NiPc and CuPc) have stronger binding strength for S8/LiPSs than metal-free phthalocyanine (H2Pc), mainly due to the strong interaction between transition metals and S atoms. Meanwhile, the formation of the M–S bond shows a weakening effect on the Li–S bonds of Li2S then boosts the decomposition of Li2S, and MPc also possesses a relatively lower lithium diffusion barrier than H2Pc. Moreover, MPc can also accelerate the multi-step conversions from S8 to Li2S by reducing the free energy of the rate-limiting reaction (Li2S2 to Li2S) in sulfur reduction reactions (SRR). Among these MPc, TiPc has the best performance in anchoring LiPSs, accelerating the decomposition of Li2S, and promoting SRR.

Unraveling the lithiophilic nature of heteroatom-doped carbons for efficient oxygen reduction in Li–O2 batteries

Nonaqueous rechargeable lithium-oxygen batteries are promising candidates of the next generation batteries. However, the sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) seriously impede their practical implementations. Herein, heteroatom-doping strategies with respect to lithiophilicity chemistry are investigated to improve the catalytic performance of carbon materials during ORR process through first-principles calculations. The binding energy, electronegativity, electric dipole moment and lithium diffusion barrier are found to be key factors that govern the lithiophilicity of doped heteroatoms. Among all non-metallic elements, pyrrolic nitrogen, carboxylic group oxygen and ketone group oxygen doped graphene nanoribbons exhibit improved thermodynamics and kinetics of ORR, giving rise to uniform deposition of lithium oxides for further favorable OER. This work provides insights into the mechanisms of ORR as well as the formation of lithium oxides and a guidance to design an effectively catalytic cathode for lithium-oxygen batteries.

Closing the gap between theory and experiment for lithium manganese oxide spinels using a high-dimensional neural network potential

Many positive electrode materials in lithium ion batteries include transition metals which are difficult to describe by electronic structure methods like density functional theory (DFT) due to the presence of multiple oxidation states. A prominent example is the lithium manganese oxide spinel Li$_x$Mn$_2$O$_4$ with $0\leq x\leq2$. While DFT, employing the local hybrid functional PBE0r, provides a reliable description, the need for extended computer simulations of large structural models remains a significant challenge. Here, we close this gap by constructing a DFT-based high-dimensional neural network potential (HDNNP) providing accurate energies and forces at a fraction of the computational costs. As different oxidation states and the resulting Jahn-Teller distortions represent a new level of complexity for HDNNPs, the potential is carefully validated by performing X-ray diffraction experiments. We demonstrate that the HDNNP provides atomic level details and is able to predict a series of properties like the lattice parameters and expansion with increasing Li content or temperature, the orthorhombic to cubic transition, the lithium diffusion barrier, and the phonon frequencies. We show that for understanding these properties access to large time and length scales as enabled by the HDNNP is essential to close the gap between theory and experiment.

Surface-Dependent Performance of Ultrathin TiN Films as an Electrically Conducting Li Diffusion Barrier for Li-Ion-Based Devices.

An in-depth understanding of lithium (Li) diffusion barriers is a crucial factor for enabling Li-ion-based devices such as three-dimensional (3D) thin-film batteries and synaptic redox transistors integrated on silicon substrates. Diffusion of Li ions into silicon can damage the surrounding components, detach the device itself, lead to battery capacity loss, and cause an uncontrolled change of the transistor channel conductance. In this study, we analyze for the first time ultrathin 10 nm titanium nitride (TiN) films as a bifunctional Li-ion diffusion barrier and current collector. Thermal atomic layer deposition (ALD) and pulsed chemical vapor deposition (pCVD) are employed for manufacturing ultrathin films. The 10 nm ALD films demonstrate excellent blocking capability with an insertion of only 0.03 Li per TiN formula unit exceeding 200 galvanostatic cycles at 3 μA/cm2 between 0.05 and 3 V versus Li/Li+. An ultralow electrical resistivity of 115 μΩ cm is obtained. In contrast, a partial barrier breakdown is observed for 10 nm pCVD films. High surface quality with low contamination is identified as a key factor for the excellent performance of ALD TiN. Conformal deposition of 10 nm ALD TiN in 3D structures with high aspect ratios of up to 20:1 is demonstrated. The measured capacities of the surface area-enhanced samples are in good agreement with the expected values. High-temperature blocking capability is proven for a typical electrode crystallization step. Ultrathin ALD TiN is an ideal candidate for an electrically conducting Li-ion diffusion barrier for Si-integrated devices.

Transition-Metal Phosphorus Trisulfides and its Vacancy Defects: Emergence of a New Class of Anode Material for Li-Ion Batteries.

In the search of suitable anode candidates with high specific capacity, favorable potential, and structural stability for lithium-ion batteries (LIBs), transition-metal phosphorus trisulfides (TMPS3 ) can be considered as one of the most promising alternatives to commercial graphite. Here, it was demonstrated that the limitations of commercial anode materials (i.e., low specific capacity, large volume change, and high lithium diffusion barrier as well as nucleation) can be circumvented by using TMPS3 monolayer surfaces. The study revealed that lithium binds strongly to TMPS3 monolayers (-2.31 eV) without any distortion of the surface, with Li@TMPS3 exhibiting enhanced stability compared with other 2D analogues (graphene, phosphorene, MXenes, transition-metal sulfides and phosphides). The binding energy of lithium was overwhelmingly enhanced with vacancy defects. The vacancy-mediated TMPS3 surfaces showed further amplification of Li binding energy from -2.03 to -2.32 eV and theoretical specific capacity of 441.65 to 484.34 mAh g-1 for MnPS3 surface. Most importantly, minimal change in volume (less than 2 %) after lithiation makes TMPS3 monolayers a very effective candidate for LIBs. Additionally, the ultralow lithium diffusion barrier (0.08 eV) compared with other existing commercial anode material proves the superiority of TMPS3 .

Are Protons Involve in the Fading Capacity in Li Ion Aqueous Batteries? Shedding Light By Means of First Principle Computations

Commercial lithium ion batteries are composed of an anode based on carbon, a transition metal oxide cathode separated by an organic liquid electrolyte containing a lithium salt such as LiPF6. Nevertheless, operating with an organic electrolyte presents many drawbacks. The organic electrolyte is flammable, expensive and need to be kept away from any water contamination. On the other hand, working in aqueous electrolyte would therefore be extremely beneficial especially to lower production cost and obtain higher safety and lower toxicity. The performance of aqueous based LIBs has been demonstrated since the foundational work from Dahn et al. who combined an anode of VO2 and a cathode of LiMn2O4 in an aqueous electrolyte.In contrast to organic electrolytes, where only the inserted metal ions can (de)intercalate in the electrode material, the use of aqueous electrolytes opens the possibility of proton co-insertion. Proton intercalation has been one of the common assumptions to explain performance issues in aqueous electrolytes such as capacity fading and the increase in lithium diffusion barriers. Several (electro)chemical studies combined with first-principles computations have confirmed the easy protonation of layered electrodes at different ranges of pH values and this fact has been directly related with the detrimental in cathode materials. Nevertheless, several studies reported the excellent performance of layered LiCoO2 and LiCo1/3Ni1/3Mn1/3O2 as positive electrode -without coating- using neutral or basic salt in water electrolytes. Thus, it remains an open question: Why these layered cathode materials so prone to proton intercalation could perform as well in aqueous environment?In this work, we use density functional computations to investigate the lithium/proton exchange in some well-known cathode materials. The analysis of the proton insertion mechanism has been carried out in order to demonstrated which specific chemical and structural features lead to electrochemically stable materials for aqueous batteries. To analyze the Li+/H+ exchange, several Li+/H+/vacancy orderings and arrangements were computed to study the phase stability at different pH values.Given that the importance of proton insertion for aqueous batteries, this study provides fundamental investigations about the role of H-intercalation in the performance of aqueous battery operations.

Multistaged discharge constructing heterostructure with enhanced solid-solution behavior for long-life lithium-oxygen batteries

Inferior charge transport in insulating and bulk discharge products is one of the main factors resulting in poor cycling stability of lithium–oxygen batteries with high overpotential and large capacity decay. Here we report a two-step oxygen reduction approach by pre-depositing a potassium carbonate layer on the cathode surface in a potassium–oxygen battery to direct the growth of defective film-like discharge products in the successive cycling of lithium–oxygen batteries. The formation of defective film with improved charge transport and large contact area with a catalyst plays a critical role in the facile decomposition of discharge products and the sustained stability of the battery. Multistaged discharge constructing lithium peroxide-based heterostructure with band discontinuities and a relatively low lithium diffusion barrier may be responsible for the growth of defective film-like discharge products. This strategy offers a promising route for future development of cathode catalysts that can be used to extend the cycling life of lithium–oxygen batteries.

Role of Long-Range Dispersion Forces in Modeling of MXenes as Battery Electrode Materials

We present a density functional theory study of stacked MXenes with the aim of assessing the accuracy of the most widely used exchange–correlation (xc) functionals for calculating properties relevant for battery electrode materials, namely, mechanical stability, open-circuit voltages, and lithium diffusion barriers. The open-circuit voltage results are nearly independent of the choice of xc-functional, while the lithium diffusion barriers are shown to be heavily dependent on the selected xc-functional, with diffusivities spanning 5 orders of magnitude. This is shown to be due to differences in the computed interlayer distances between the MXene monolayers, which are found to be linearly related to the diffusion barriers. This underlines the importance of starting from reliable crystal structures when modeling MXenes as battery electrode materials. Using an experimental reference materials database, we show a general trend in lattice parameter accuracy for 2D stacked materials with the chosen xc-functional...

Understanding the roles of Ti on the structure and electrochemical performances of Li2Ru1-Ti O3 cathode materials for Li-ion batteries

Abstract The lattice doping has been widely used to improve the electrochemical performances of Li-rich cathode materials but the roles of the introduced foreign atoms are still not very clear. Herein, a series of Li2Ru1-xTixO3 solid solutions have been synthesized and the roles of Ti doping on the structural and electrochemical properties of Li2RuO3 have been comprehensively investigated. The Rietveld refinement exhibits that the interlayer spacing gradually shortens with increasing Ti content. This shrinkage is favorable to the layered structure stability but increases the lithium diffusion barrier. Galvanostatic measurements show that Li2Ru0.8Ti0.2O3 possesses the best cyclability with 196.9 and 196.1 mAh g−1 for charge and discharge capacity retaining after 90 cycles, respectively. Cyclic voltammetry scanning indicates that Ti dopant promotes the formation of more peroxo- or superoxo-like species but reduces the initial coulumbic efficiency. Results of electrochemical impedance spectroscopy display that Ti doping reduces the charge transfer impedance, which facilitates the lithium-ion diffusion across the electrolyte-electrode interface and improves the electronic conductivity. Li2Ru0.8Ti0.2O3 exhibits the best electrochemical performance owing to the balance among all the factors discussed above. This study also offers some new insights into optimizing the electrochemical performances of Li-rich cathode materials through the lattice doping.

Interfacial strain effects on lithium diffusion pathways in the spinel solid electrolyte Li-doped MgAl2O4

(Li,Al)-co-doped magnesium spinel (Li$_x$Mg$_{1-2x}$Al$_{2+x}$O$_4$) is a solid lithium-ion electrolyte with potential use in all-solid-state lithium-ion batteries. Interfaces with spinel electrodes, such as Li$_y$Mn$_2$O$_4$ and Li$_{4+3z}$Ti$_5$O$_{12}$, may be lattice-matched, with potentially low interfacial resistances. Small lattice parameter differences across a lattice-matched interface are unavoidable, causing residual epitaxial strain. This strain potentially modifies lithium diffusion near the interface, contributing to interfacial resistance. Here we report a density functional theory study of strain effects on lithium diffusion pathways for (Li,Al)-co-doped magnesium spinel, for $x_\mathrm{Li} = 0.25$ and $x_\mathrm{Li} = 0.5$. We have calculated diffusion profiles for the un-strained materials, and for isotropic and biaxial tensile strains of up to 6%, corresponding to {100} epitaxial interfaces with Li$_y$Mn$_2$O$_4$ and Li$_{4+3z}$Ti$_5$O$_{12}$. We find that isotropic tensile strain reduces lithium diffusion barriers by as much as 0.32 eV, with typical barriers reduced by ~0.1 eV. This effect is associated with increased volumes of transitional octahedral sites, and broadly follows local electrostatic potentials. For biaxial (epitaxial) strain changes in octahedral site volumes and in lithium diffusion barriers are much smaller than under isotropic strain. Typical barriers are reduced by only ~ 0.05 eV. Individual effects, however, depend on the pathway considered and the relative strain orientation. These results predict that isotropic strain strongly affects ionic conductivities in (Li,Al)-co-doped magnesium spinel electrolytes, and that tensile strain is a potential route to enhanced lithium transport. For a lattice-matched interface with candidate spinel-structured electrodes epitaxial strain has a small, but complex, effect on lithium diffusion barriers.

Lithium Trapping in Alloy Forming Electrodes and Current Collectors for Lithium Based Batteries

The next generation of lithium based batteries can be expected to be based on lithium alloy forming anode materials which can store up to ten times more charge than the currently used graphite anodes. This increase in the charge storage capability has motivated significant research towards the commercialization of anode materials such as Si, Sn and Al. These alloy forming anode materials are, however, known to exhibit significant capacity losses during cycling. These are typically ascribed to the volume expansion associated with the formation of the lithium alloys (the volume expansion is e.g. about 280 % for Li3.75Si) resulting in electrode pulverization as well as continuous solid electrolyte interphase (SEI) layer formation [1-3]. While significant progress has been made to decrease the volume expansion problems by the use of e.g. nanoparticles, nanorods and thin films, and/or capacity limitations [1-3], capacity losses are still generally seen [4,5]. This and previously published data suggest that the phenomenon may be due to another effect, possibly as a result of lithium trapping in the electrodes [6-8]. In the present work it is demonstrated (based on e.g. elemental analyses of cycled Sn, Al and Si electrodes) that lithium trapping can account for the capacity losses seen when alloy forming anode materials are cycled versus lithium electrodes, see Figure 1. It is shown that small amounts of elemental lithium are trapped within the electrode material during the cycling as a result of a two-way diffusion process [8] causing the lithium to move into the bulk material even during the delithiation step. This phenomenon, which can be explained by the lithium concentration profiles in the electrodes, makes a complete delithiation process very time consuming. As a result of the lithium trapping effect, the lithium concentration in the electrode increases continuously during the cycling. The experimental results also show that a similar effect can be seen also for commonly used current collector metals such as Cu, Ni and Ti. The latter means that these metals are unsuitable as current collector materials for lithium alloy forming materials in the absence of a thin layer of boron doped diamond serving as a lithium diffusion barrier layer [8]. References 1 M. N. Obrovac and V. L. Chevrier, Chem. Rev., 2014, 114, 11444. 2 X. Su, Q. Wu, J. Li, X. Xiao, A. Lott, W. Lu, B. W. Sheldon and J. Wu, Adv. Energy Mater., 2014, 4, 1300882. 3 J. R. Szczech and S. Jin, Energy Environ. Sci., 2011, 4, 56. 4 G. Zheng, S. W. Lee, Z. Liang, H-W. Lee, K. Yan, H. Yao, H. Wang, W. Li, S. Chu and Y. Cui, Nat. Nanotechnol., 2014, 9, 618. 5 K. Yan, H-W. Lee, T. Gao, G. Zheng, H. Yao, H. Wang, Z. Lu, Y. Zhou, Z. Liang, Z. Liu, S. Chu and Y. Cui, Nano Letters, 2014, 14, 6016. 6 G. Oltean, C-W. Tai, K. Edström and L. Nyholm, J. Power Sources, 2014, 269, 266. 7 A. L. Michan, G. Divitini, A. J. Pell, M. Leskes, C. Ducati and C. P. Grey, J. Am. Chem. Soc., 2016, 138, 7918. 8 D. Rehnlund, F. Lindgren, S. Böhme, T. Nordh, Y. Zou, J. Pettersson, U. Bexell, M. Boman, K. Edström and L. Nyholm, Energy Environ. Sci., 10 (2017) 1350. Figure 1