Theoretical evaluation of high-energy lithium metal phosphate cathode materials in Li-ion batteries

Lithium metal phosphates (olivines) are emerging as long-lived, safe cathode materials in Li-ion batteries. Nano-LiFePO₄ already appears in high-power applications, and LiMnPO₄ development is underway. Current and emerging Fe- and Mn-based intercalants, however, are low-energy producers compared to Ni and Co compounds. LiNiPO₄, a high voltage olivine, has the potential for superior energy output (>10.7 Wh in 18650 batteries), compared with commercial Li(Co,Ni)O₂ derivatives (up to 9.9 Wh). Speculative Co and Ni olivine cathode materials charged to above 4.5 V will require significant advances in electrolyte compositions and nanotechnology before commercialization. The major drivers toward 5 V battery chemistries are the inherent abuse tolerance of phosphates and the economic benefit of LiNiPO₄: it can produce 34% greater energy per dollar of cell material cost than LiAl_0.05Co_0.15Ni_0.8O₂, today's “standard” cathode intercalant in Li-ion batteries.     

Thermal safety study of Li-ion batteries under limited overcharge abuse based on coupled electrochemical-thermal model

Many fire accidents of electric vehicles were reported that happened during the charging process. In order to investigate the reasons that lead to this problem, this paper studies the thermal safety of Li-ion batteries under limited overcharge abuse. A 3D electrochemical-thermal coupled model is developed for modeling thermal and electrochemical characteristics from normal charge to early overcharge state. This model is validated by experiment at charge rates of 0.5C, 1C, and 2C. The simulation results indicate that irreversible heat contributes most to temperature rise during the normal charge process, but the heat induced by Mn dissolution and Li deposition gradually dominates heat generation in the early overcharge period. Based on this, a threshold selection method for multistage warning of batteries overcharge is proposed. Among them, level 1 should be considered as a critical stage during the early overcharge process due to the deposited lithium starts to react with electrolyte at the end of level 1, where temperature rate increases to 0.5°C min⁻¹ for 1C charge. While the thresholds of levels depend on charge rate and composition of battery. Furthermore, several critical parameters are analyzed to figure out their effects on thermal safety. It is found that the temperature at the end of overcharge is significantly influenced by the change of positive electrode thickness and solid electrolyte interface (SEI) film resistance. The final temperature increases by 17.5°C and 7.9°C, respectively, with positive electrode thickness ranging from 50 to 80 μm and SEI film resistance increasing from 0.002 to 0.03 Ω.     

SnSb micron-sized particles for Li-ion batteries

Micrometre-sized particles of Sn/SnSb were produced with a simple technique consisting in melting commercial ingots of tin and antimony separately at 280 °C and 680 °C, respectively, and casting them together in a ceramic boat. The solid alloy was then crushed into a homogeneous powder by grinding and sieving. The obtained powder was characterised by X-ray diffraction, and electron microscopy. Elemental and phase composition analyses were performed via, inductive coupled plasma and differential scanning calorimetry, respectively. The material was further tested as electrode material in a lithium galvanic cell. It showed relatively good capacity retention for at least 15 cycles. TEM analysis on post-mortem electrode samples showed the formation of nanostructures after the first discharge followed by a progressive disappearance of the micron-sized particles upon further cycling. Fading at higher cycles is explained by the formation of isolated metallic nano-particles that become inactive for further storage of lithium.     

An inorganic composite membrane as the separator of Li-ion batteries

We studied an inorganic composite membrane as the separator for Li-ion batteries. Being made of mainly CaCO₃ powder and a small amount of polymer binder, the composite membrane has excellent wettability with liquid electrolytes due to its high porosity and good capillarity. Ionic conductivity of the membrane can be easily achieved by absorbing a liquid electrolyte. Additional benefit of such a membrane is that the alkali CaCO₃ can scavenge acidic HF, which is inevitably present in the LiPF₆-based electrolytes used currently in the Li-ion batteries. In this work, we typically evaluated a membrane with the composition of 92:8 (wt.) CaCO₃/Telfon by using a 1.0 m LiPF₆ dissolved in a 3:7 (wt.) mixture of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) as the liquid electrolyte. Ionic conductivity of the electrolyte-wetted membrane was measured to be 2.4 mS cm⁻¹ at 20 °C versus 8.0 mS cm⁻¹ of the liquid electrolyte. With the said membrane as a separator, both Li/graphite and Li/cathode half-cells exhibited good capacity retention. We also found that the Li-ion cell fabricated in this manner not only had stable capacity retention, but also showed good high-rate performance.     

Understanding the characteristics of high-voltage additives in Li-ion batteries: Solvent effects

Calculations are made of the ionization potential (IP) and the oxidation potential (E_ox) values of 108 organic molecules that are potential electrolyte additives for the overcharge protection of lithium-ion batteries (LIBs). The calculated E_ox values are in close agreement with the experimental ones, where the root-mean-square deviation is 0.08 V and the maximum deviation is 0.15 V. The molecules exhibiting high E_ox (>4.5 V) show one of the following two features: (1) IP > 7.70 eV or (2) IP < 7.70 eV with a relatively large molecule size. Consideration of bulk solvent effects, in particular the electrostatic attraction between solute and solvent, is crucial in determining E_ox. Considering its accuracy and reliability, the density functional calculation is recommended as a useful tool for screening electrolyte additives for LIBs.     

Effect of electrolytes on the structure and evolution of the solid electrolyte interphase (SEI) in Li-ion batteries: A molecular dynamics study

We have studied the formation and growth of solid-electrolyte interphase (SEI) for the case of ethylene carbonate (EC), dimethyl carbonate (DMC) and mixtures of these electrolytes using molecular dynamics simulations. We have considered SEI growth on both Li metal surfaces and using a simulation framework that allows us to vary the Li surface density on the anode surface. Using our simulations we have obtained the detailed structure and distribution of different constituents in the SEI as a function of the distance from the anode surfaces. We find that SEI films formed in the presence of EC are rich in Li₂CO₃ and Li₂O, while LiOCH₃ is the primary constituent of DMC films. We find that dilithium ethylene dicarbonate, LiEDC, is formed in the presence of EC at low Li surface densities, but it quickly decomposes to inorganic salts during subsequent growth in Li rich environments. The surface films formed in our simulations have a multilayer structure with regions rich in inorganic and organic salts located near the anode surface and the electrolyte interface, respectively, in agreement with depth profiling experiments. Our computed formation potentials 1.0 V vs. Li/Li⁺ is also in excellent accord with experimental measurements. We have also calculated the elastic stiffness of the SEI films; we find that they are significantly stiffer than Li metal, but are somewhat more compliant compared to the graphite anode.     

Thermal safety studies of high energy density lithium-ion batteries under different states of charge

The popularity of lithium-ion batteries in electric vehicles has promoted the increase of its energy density, and battery cathode and anode materials have developed rapidly in recent years. As the next generation of material systems, high-nickel-content Li-Ni-Co-Mn oxide cathode and high-silicon-content Si-C anode material systems have a high potential for further application. However, safety is a key indicator for their use in traction batteries. We thus conducted a thermal safety analysis of the pouch cells of such a system for different states of charge and revealed the key factors for the thermal safety evolution of batteries by analyzing the morphology and thermal stability of cathodes and anodes.     

Constructing Janus SnSSe and graphene heterostructures as promising anode materials for Li-ion batteries

Developing efficient anode materials for Li-ion batteries is becoming increasingly important but is still challenging to collect relevant information about their adsorption and diffusion. Herein, by means of density functional theory (DFT) computations, the Janus SnSSe, and graphene van der Waals heterostructures (ie, SSnSe/G and SeSnS/G) are systematically investigated by first principles calculations, aiming at constructing promising anode materials for Li-ion batteries (LIBs). The results have demonstrated that the SnSSe/G heterostructures exhibits a semimetal-to-metal transition after incorporating Li, indicating enhanced conductivity compared to monolayer Janus SnSSe or graphene. Moreover, the SnSSe/G heterostructures can maintain favorable structural stability and ultrahigh stiffness well after applying the strain or adsorption of lithium atoms, thereby ensuring the pulverization resistance. In addition, the energy barriers of Li atoms diffusion are very low, which are expected to achieve a fast charge/discharge rate. Meanwhile, the estimated storage capacity of Li on SnSSe/G heterostructures could achieve 472.66 mA h g^?1, which greatly improves the storage capacity. These interesting results show that Janus SnSSe/G heterostructures could be used as excellent anode materials for LIBs.     

Low Li binding affinity: An important characteristic for additives to form solid electrolyte interphases in Li-ion batteries

Calculations are made of the lowest unoccupied molecular orbital (LUMO), chemical hardness (η), dipole moment (μ), and binding energy with a Li⁺ ion for 32 organic molecules that are electrolyte additives for solid electrolyte interphase (SEI) formation in lithium-ion batteries (LIBs). The results confirm that both the LUMO and η values are critical indicators of suitable SEI formation. The μ values of the additives are generally smaller than those of widely used solvents in LIBs. It is found that a low Li-ion binding affinity may be an important characteristic for SEI-forming additives. Li⁺ binding affinity is proposed as a factor in the computational screening process used to obtain promising additives.     

Screening Li-ion batteries for internal shorts

A Li-ion polymer pouch cell battery design for a spacesuit developed an internal short during ground storage. A detailed failure investigation found that native contamination was the most probable root cause as the failure mechanism was successfully replicated. Lessons learned are applicable to the implementation of most Li-ion cell designs for critical applications.     

Sol-gel synthesis of multiwalled carbon nanotube-LiMn2O4 nanocomposites as cathode materials for Li-ion batteries

This study reports the development of multiwalled carbon nanotube (MWCNT)-LiMn₂O₄ nanocomposites by a facile sol-gel method. The elemental compositions, surface morphologies and structures of the nanocomposites are characterized with a view to their use as cathode materials for Li-ion batteries. The results indicate that the nanocomposite consists of LiMn₂O₄ nanoparticles containing undamaged MWCNTs. The nanocomposites show high cycle performance with a remarkable capacity retention of 99% after 20 cycles, compared with LiMn₂O₄ nanoparticles with a 9% loss of the initial capacity after 20 cycles. Measurements of a.c. impedance show that the charge-transfer resistance of the nanocomposites is much lower than that of spinel LiMn₂O₄. A cyclic voltammetry study further confirms higher reversibility of the nanocomposites compared with LiMn₂O₄ particles. The enhanced electrochemical performance of the nanocomposites is attributed to the formation of conductive networks by MWCNTs that act as intra-electrode wires, thereby facilitating charge-transfer among the spinel LiMn₂O₄ particles.     

Progress of electrolyte additives for high-capacity power lithium ion batteries

动力电池是新能源汽车的核心部件,而电解液是制约动力电池发展的关键。电解液一般由碳酸酯类溶剂、锂盐和添加剂组成,其性质对电池的高低温、倍率、寿命等性能有显著影响。高比能动力电池所需电解液的主要开发策略是利用功能添加剂在电池正、负极同时形成稳定的保护膜,同时稳定界面。文章回顾了近年来匹配高压正极材料和高容量硅碳负极材料所需添加剂的组成和基本功能,论述了添加剂作用机理和发展趋势,认为300 W·h/Kg高能量密度电池电解液的关键在于开发新型多功能添加剂。     

LiFePO4 water-soluble binder electrode for Li-ion batteries

A new water-soluble elastomer from ZEON Corp. was evaluated as binder with LiFePO₄ cathode material in Li-ion batteries. The mechanical characteristic of this cathode was compared to that with PVdF-based cathode binder. The elastomer-based cathode shows high flexibility with good adhesion. The electrochemical performance was also evaluated and compared to PVdF-based cathodes at 25 and at 60 °C. A lower irreversible capacity loss was obtained with the elastomer-based cathode, however, aging at 60 °C shows a comparable cycle life to that observed with PVdF-based cathodes. The LiFePO₄–WSB at high rate shows a good performance with 120 mAh g⁻¹ at 10C rate at 60 °C.     

Preparation and characterization of core-shell battery materials for Li-ion batteries manufactured by substrate induced coagulation

In this work Substrate Induced Coagulation (SIC) was used to coat the cathode material LiCoO₂, commonly used in Li-ion batteries, with fine nano-sized particulate titania. Substrate Induced Coagulation is a self-assembled dip-coating process capable of coating different surfaces with fine particulate materials from liquid media. A SIC coating consists of thin and rinse-prove layers of solid particles. An advantage of this dip-coating method is that the method is easy and cheap and that the materials can be handled by standard lab equipment. Here, the SIC coating of titania on LiCoO₂ is followed by a solid-state reaction forming new inorganic layers and a core-shell material, while keeping the content of active battery material high. This titania based coating was designed to confine the reaction of extensively delithiated (charged) LiCoO₂ and the electrolyte. The core-shell materials were characterized by SEM, XPS, XRD and Rietveld analysis.     

Recent progress of electrode materials for room-temperature sodium-ion stationary batteries

随着风能、太阳能等可再生能源的不断发展,储能作为影响其发展的关键技术越来越受到人们的关注。在储能领域,锂离子电池以高能量密度、长循环寿命、高电压等诸多优点在电子领域已得到广泛的应用,并成为未来电动汽车动力电池的最佳选择。但因锂资源储量有限、分布不均匀,而且原材料成本比较高,所以锂离子电池在电网大规模储能方面的应用遇到了瓶颈。与锂相比,钠不但具有与锂相似的物理化学性质,更具有资源丰富、分布广泛、原料成本低廉等优势。近些年室温钠离子电池再次引起了人们的研究兴趣,特别是在电网储能方面表现出极大的应用潜力。虽然目前已报道了多种钠离子电池电极材料,但大都离实用化以及进一步产业化尚有一定的距离。本文重点介绍一些性能较为突出的室温钠离子电池电极材料,并指出要实现钠离子电池的产业化,需要开发空气中稳定、高安全、高容量、高倍率、循环稳定、低成本的新型正、负极材料。     

Effect of activation at elevated temperature on Li-ion batteries with flame-retarded electrolytes

The effect of activation temperature on Li-ion batteries with flame-retarded electrolytes containing 5 wt.% dimethyl methyl phosphonate (DMMP) and trimethyl phosphate (TMP) is investigated respectively. It is found that activation at elevated temperature promotes the formation of a stable solid electrolyte interface layer on the graphite electrode, which may significantly suppress the reductive decomposition of DMMP and TMP and avoid graphite exfoliation. But fierce oxidation of the electrolytes on the LiCoO₂ electrode at elevated temperature is harmful to the cell performance. A procedure of so-called altered temperature activation (ATA) is adopted for LiCoO₂/graphite full-cells. It can compromise the contradictive effects on the separate electrodes at the elevated temperature. High capacity and good rate capability are obtained for the cells with the flame-retarded electrolytes, especially for the TMP-containing electrolyte.     

Discrimination of Li-ion batteries based on Hamming network using discharging-charging voltage pattern recognition for improved state-of-charge estimation

Differences in electrochemical characteristics among Li-ion batteries and factors such as temperature and ageing result in erroneous state-of-charge (SoC) estimation when using the existing extended Kalman filter (EKF) algorithm. This study presents an application of the Hamming neural network to the identification of suitable battery model parameters for improved SoC estimation. The discharging-charging voltage (DCV) patterns of ten fresh Li-ion batteries are measured, together with the battery parameters, as representative patterns. Through statistical analysis, the Hamming network is applied for identification of the representative DCV pattern that matches most closely of the pattern of the arbitrary battery to be measured. Model parameters of the representative battery are then applied to estimate the SoC of the arbitrary battery using the EKF. This avoids the need for repeated parameter measurement. Using model parameters selected by the proposed method, all SoC estimates (off-line and on-line) based on the EKF are within ±5% of the values estimated by ampere-hour counting.     

Application of bis(fluorosulfonyl)imide-based ionic liquid electrolyte to silicon-nickel-carbon composite anode for lithium-ion batteries

An ionic liquid electrolyte containing bis(fluorosulfonyl)imide (FSI) anion without any solvent is applied to a silicon-nickel-carbon (Si-Ni-carbon) composite anode for rechargeable lithium (Li)-ion batteries. The FSI-based ionic liquid electrolyte successfully provides a stable, reversible capacity for the Si-Ni-carbon anode, which is comparable to the performance observed in a typical commercialized solvent-based electrolyte, while a common ionic liquid electrolyte containing bis(trifluoromethanesulfonyl)imide (TFSI) anion without FSI presents no reversible capacity to the anode at all. Ac impedance analysis reveals that the FSI-based electrolyte provides very low interfacial and charge-transfer resistances at the Si-based composite anode, even when compared to the corresponding resistances observed in a typical solvent-based electrolyte. Galvanostatic cycling of the Si-based composite anode in the FSI-based electrolyte with a charge limitation of 800 mAh g⁻¹ is stable and provides a discharge capacity of 790 mAh g⁻¹ at the 50th cycle, corresponding to a cycle efficiency of 98.8%.     

Enhanced electrochemical properties of LiFePO4 cathode for Li-ion batteries with amorphous NiP coating

Here we show that the intrinsic low electrical conductivity of LiFePO₄ which seriously hinders the application of LiFePO₄ for Li-ion batteries is overcome with conductive metallic NiP nano-coating. High resolution transmission electron microscopy image reveals that the NiP coating is a nanoscale amorphous layer, which was deposited on the LiFePO₄ particles to form a so-called core/shell structure via electroless plating at room temperature. The electrochemical performances of NiP coated LiFePO₄ show that both of the rate performance and cycleability of LiFePO₄ against graphite anode are improved by the NiP coating. Analysis of electrochemical impedance spectra of the LiFePO₄/graphite cells demonstrates that the NiP coating decreases both of the surface film resistance and charge transfer resistance. The dissolution of Fe from LiFePO₄ in the LiPF₆ based electrolyte is remarkably suppressed by the protective NiP coating.     

全固态锂离子电池关键材料研究进展

全固态锂离子电池采用固态电解质替代传统有机液态电解液,有望从根本上解决电池安全性问题,是电动汽车和规模化储能理想的化学电源。为了实现大容量化和长寿命,从而推进全固态锂离子电池的实用化,电池关键材料的开发和性能的优化刻不容缓,主要包括制备高室温电导率和电化学稳定性的固态电解质以及适用于全固态锂离子电池的高能量电极材料、改善电极/固态电解质界面相容性。本文以全固态锂离子电池关键材料为出发点,综述了不同类型的固态电解质和正负极材料性能特征以及电极/电解质界面性能的调控和优化方法等,阐述了未来全固态锂离子电池关键材料的发展方向以及界面问题的解决思路,为探索全固态锂离子电池产业化前景奠定基础。