Lithium-ion battery: It is a secondary battery (rechargeable battery) that mainly relies on the movement of lithium ions between the positive and negative electrodes of the battery. That is, during the charge and discharge process, Li+ goes back and forth between the two electrodes to perform the intercalation and deintercalation process. Among them, during charging, Li+ is deintercalated from the positive electrode and embedded in the negative electrode through the electrolyte. The negative electrode is in a lithium-rich state; while during discharge, the opposite is true. Lithium-ion batteries have the following advantages: 1) High voltage, the working voltage of a single battery is as high as 3.7-3.8 V; 2) Large specific energy; 3) Long cycle life; 4) Good safety performance, no pollution, no pollution Memory effect; 5), small self-discharge; 6), fast charging; 7), working temperature, the working temperature is generally between 25~45oC. However, macroscopically, lithium-ion batteries have the following main disadvantages: 1) Aging and limited service life; 2) Low recovery rate; 3) Intolerance to overcharging; because during overcharging, excess embedded lithium ions will be permanently fixed In the crystal lattice, it cannot be released again, which can lead to shortened battery life. 4) Intolerance to over-discharge; because during over-discharge, the electrode will deintercalate too many lithium ions, which can cause the crystal lattice to collapse, thereby shortening the life. Therefore, when we use electronic products equipped with lithium-ion batteries, we should pay attention to some matters: when charging, it should not be higher than the maximum charging voltage, and when discharging, it should not be lower than the minimum operating voltage; do not frequently deep discharge or deep charge; avoid high temperature, light Otherwise, the lifespan will be shortened, and in severe cases, it may cause an explosion; lithium-ion batteries will also naturally age when not in use. Lithium-ion batteries play an indispensable role in our lives, so what are the latest developments in lithium-ion batteries? What methods are currently available to overcome or avoid its own shortcomings?

　　Latest research progress

　　Since their successful commercialization in the 1990s, lithium-ion batteries (LIBs) have been widely used in portable digital products. However, the energy density and power density of many existing LIBs are insufficient to meet the current growing demands.

　　Therefore, considering the cost distribution of battery systems, it is crucial to explore anode/cathode materials with excellent rate performance and long cycle life. Although nanoscale electrode materials can quickly absorb and store large amounts of Li+ due to short diffusion channels and large surface areas, the low thermodynamic stability of nanoparticles leads to electrochemical agglomeration and increases the risk of side reactions on the electrolyte. The following methods can improve the above shortcomings.

　　1. Methods to increase energy density and power density from negative electrode materials

　　(1) Multi-layer self-assembly structure realizes the advantages of integrating materials of different sizes

　　In layered transition metal oxides, Li and M (M = metal) cations occupy the octahedral voids of the O-array. The Li layer is located between two adjacent MO6 octahedral layers, and Li ions have a two-dimensional (2D) diffusion path. For example, the following practical example:

　　Ni-rich layered transition metal oxides

　　Ni-rich layered transition metal oxides are derived from high-capacity LiNiO2. Since the energy band of e.g. redox-active Ni4+/Ni3+ only slightly overlaps the top of the 2p band O2 ions in Li1-xNiO2, LiNiO2 can achieve a capacity of approximately 200 mA hg-1 when cycled in the following range. However, LiNiO2 suffers from non-stoichiometric structure, structural degradation, and capacity fading due to the migration of Ni3+ ions into the Li layer. To improve thermal stability and improve performance degradation, cation-substituted layered transition metal oxides as well as structural doping have been studied, as follows.

　　(2) The synergistic effect provided by the core/yolk-shell structure

　　In addition to LiFePO4 and LiMnPO4, LiFexMn1-xPO4 is also a promising anode material. For example, Scrosati and colleagues prepared carbon-coated core-shell structure LiMn0.85Fe0.15PO4-LiFePO4 through a two-step precipitation route, which combines the high potential of LiMnPO4 and the high stability of LiFePO4.

　　(3) The porous structure of macropores, mesopores and micropores adapts to volume expansion and promotes electrolyte penetration.

　　The unique layered structure has a network of electrolyte-expanded macro/mesopores and a buffered protective carbon shell, which facilitates continuous electron conduction and rapid ion transport. For example, the following example:

　　Although Li3V2(PO4)3 has higher electronic conductivity (≈10-7S cm-1) than LiFePO4 and LiMnPO4, this value is still very low, severely limiting its power density. Mai and co-workers fabricated bicontinuous layered Li3V2(PO4)3/C mesoporous nanowires through hydrothermal and annealing treatments. The hierarchical structure endows Li3V2(PO4)3/C nanowires with enhanced rate performance and cycling stability. When cycled between 3.0 and 4.3 V, the composite achieved high rate performance and ultra-long-term cyclability (capacity retention of 80.0% after 3000 cycles). There is an electrolyte-expanded macro/mesoporous network and a buffered protective carbon shell in the unique layered structure, which facilitates continuous electron conduction and rapid ion transport.

　　(4) Change the negative electrode material of lithium-ion battery

　　For example, the Chem. Soc. Rev. review recently published by the team of Professor Mai Liqiang and Professor Zhou Liang summarized in detail silicon oxide as a promising anode material for lithium-ion batteries.

　　2. Methods to improve energy density and power density from cathode materials

　　(1) Nanoengineering technology to enhance conversion cathode materials (CTAM)

　　"Conversion reaction" usually refers to the redox reaction between Li+ and transition metal compounds (MaXb, M = Mn, Fe, Co, Ni, Cu, X = O, S, Se, F, N, P, etc.). It involves the formation and decomposition of lithium binary compounds (LinX) with high theoretical specific capacities (Eq. 1). Typically, the reaction potential determined by the ionicity of the M-X bond is in the range of 0.5-1.0 V relative to Li/Li+, making most transition metal compounds useful as potential cathodes.

　　MaXb+ (b.n)Li++ (b.n)e-aM + b LinX (Equation 1)

　　In this equation, the formation of LinX is thermodynamically feasible. However, it is difficult to decompose electrochemically inert LinX by bulk M powder. Therefore, the key to the reversibility of this conversion mechanism lies in the formation of highly electroactive M nanoparticles to decompose the LinX matrix surrounded by a solid electrolyte interphase (SEI) layer. Furthermore, the voltage hysteresis appears to be highly dependent on the nature of the anionic species in the conversion cathode material (CTAM), decreasing in the order of fluoride > oxide > sulfide > nitride > phosphide.

　　Use nanoengineering technology to enhance conversion-type cathode materials (CTAM) to increase the energy density of lithium-ion batteries. Mainly include the use of low-dimensional nanostructures, hierarchical porous nanostructures, hollow structures and hybridization with various carbonaceous materials.

　　3. Utilize core double-shell electrodes to promote high gravimetric energy density of flexible lithium-ion batteries

　　Although the reported flexible materials have excellent properties, their main problem is the degree of mechanical stability. Although the excellent mechanical stability of carbon fiber cloth (CC) can solve this problem, CC is still limited by low surface area, greater weight, and low storage capacity. As reported by Tong's group, NiCo2O4 nanowires (NCO NWs) were grown on flexible CC core-shell anodes (CC@EC) to design monolithic core-double shell (CDS@NCO CDS). CC@EC@ NCO CDS electrode shows better lithium storage performance than pristine CC-coated NCO (CC@NCO).

　　4. Stability research

　　For example, Shi and Koratka's research group used van der Waals sliding interfaces to improve the electrochemical stability of silicon film anodes in lithium-ion batteries. That is, better electrochemical stability is obtained by designing a van der Waals "smooth" interface between the Si film and the current collector. This is achieved by simply coating graphene sheets on the surface of the current collector. The interface formed, the Si film slides relative to the current collector under the action of lithiation/delithiation while maintaining electrical contact with the current collector. Electrochemical tests confirmed the more stable performance and higher Coulombic efficiency of Si films deposited on graphene-coated nickel (smooth interface).

　　5. Security model research

　　The rapid growth of battery energy density, accompanied by the significant reduction in the cost of lithium-ion batteries, has brought safety issues. Although the more energy stored in the battery pack of an electric vehicle, the longer the driving range, the more serious the accident will be because the battery will explode. Therefore, the safety issues of lithium-ion batteries are receiving more and more attention. Research on battery safety spans multiple scales. Scale-specific and scale-specific progress are discussed below in three parts.

　　(1) Microscale and mesoscale: basic models of battery components

　　The electrode stack of commercial lithium-ion batteries currently on the market is a multi-layer structure, and a single repeatable unit consists of a negative electrode, a positive electrode and two layers of separators. In addition, the negative electrode is made of aluminum foil, which is coated with active material and adhesive on both sides. Likewise, the positive electrode consists of copper foil coated with graphite (or silicon) particles. All components are immersed in electrolyte and covered with a pouch or steel shell casing. The chemistry and materials of the components may differ from battery manufacturer to battery manufacturer, but the basic structure of this repeatable unit is nearly the same.

　　(2) Macroscale or unit level

　　On a macro scale, a battery is a complex assembly consisting of a current collector, active coating material, separator, and casing. The mechanical performance of a battery cell is not simply the sum of the contributions of each component. Through mechanical abuse testing, battery cells should be tested under different load conditions to obtain information on the deformation mechanism and structural response, which can be used to establish deformation models and calibration procedures. Different strategies for modeling are currently commonly used: 1), detailed models; 2), representative volume element (RVE) method; 3), development of homogeneous models. The detailed model includes the most information about the real battery cells, with the homogenized model being the most computationally efficient and the RVE method falling in between these two strategies.

　　(3) Macro system scale: battery module and protection structure

　　How to integrate rich information from individual cells into computational models of modules and battery packs is crucial to the automotive industry. On the experimental side, there are many effects (side or bottom) as well as different module designs. Therefore, the cost of a comprehensive program of crush testing will be very time-consuming and expensive. There are few publications on abuse testing of battery modules, covering only limited shapes and loading conditions. Additionally, modules are tested using 30% and 100% SOC. What is measured in the test is the time history of load, displacement and voltage.

　　Application areas and urgent problems to be solved

　　1. Portable electronic equipment

　　Portable electronic devices include tablets, laptops, digital cameras, camcorders, toys, etc. In terms of battery market share, mobile phones, tablets and laptops are by far the main applications. Currently, portable electronics require miniaturization of batteries while maintaining high capacity and power and still complying with strict safety standards. Therefore, while LCO will continue to be the mainstream chemical for portable electronics in the short term, it will gradually lose market share to NMC and NCA batteries.

　　2. Transportation

　　Warranties for electric vehicle batteries currently require calendar life and total driving distance to be considered. The last one converts to a limited number of complete cycles. Most EV manufacturers are responsible for 500-800 complete cycles and a calendar life of about 8 years. The trend of increasing capacity of new generation car batteries makes warranty conditions better. In practical applications, studying how to extend the cycle life of lithium-ion batteries is still of certain significance for electric vehicles, and for plug-in hybrid vehicles and electric motorcycles. and electric bicycles still require lithium-ion batteries with high specific energy and power and long cycle life.

　　3. Power supply system

　　Power supply systems include grid-connected systems and off-grid systems. In the power grid, the power supply must be balanced with user demand under strict quality standards, i.e. it must ensure that the nominal values of the uninterruptible power supply (such as frequency and voltage) have a small margin. The table below outlines the cost of energy storage for lithium-ion batteries, taking into account different specific costs and cycle life, for one and two cycles per day of use. The results in the table are divided into three color areas. Dark gray indicates that energy storage costs above 15 c€/kWh are not competitive for grid-connected use. Light gray corresponds to a cost range of 10-15 c€/kWh, which may bring competitiveness in the case of drastic changes in electricity prices, especially if the lithium-ion battery can also guarantee ancillary services such as operating reserves. White represents the competitive cost range below 10 c€/kWh. Of course, beyond these general patterns, closer evaluation under actual market conditions is needed to determine how competitive such storage charges are and the profitability that can be expected from such an investment. Clearly, lithium-ion batteries are still far from being cost-competitive for grid-connected use as highlighted in the table below.

　　Summarize

　　Current research on lithium-ion batteries is still mainly focused on the improvement of materials to increase the energy density and power density of the battery. For anode materials, microstructures assembled from nanoparticles combined with surface modification provide improved structural stability and rate performance. Core-shell or concentration gradient structures exhibit high capacity with high capacity retention. Nanocomposites of lithiated transition metal phosphates/silicates and carbon materials exhibit enhanced conductivity and cycling stability. In terms of cathode materials, Si/C, Sn/C and Ge/C composites with embedded structures, porous Li4Ti5O12/C composites and multi-shell hollow metal oxides all have high rate and cycle performance. In fact, each material has its own advantages and disadvantages. Combining the advantages of the corresponding materials with the rational design of the structure and the use of more advanced methods can effectively improve the electrochemical performance of LIBs anode and cathode materials, which will be better. Serve life.

　　Original title: Know it or not: the latest research progress of lithium-ion batteries

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