Can not stop the pace: high specific energy lithium-ion battery

The birth of lithium-ion batteries can be said to be a revolution in the field of energy storage, the widespread use of lithium-ion batteries completely changed our lives, light mobile phones, laptops, long-life electric vehicles, etc. Our lives have been Lithium-ion batteries tightly tied together, it is difficult to believe that if we lost like lithium-ion battery so convenient and efficient energy storage battery after our life will become what kind of. With the continuous development of lithium-ion battery technology, we also made a higher demand for lithium-ion battery performance, we hope that lithium-ion battery smaller, more lightweight, more energy storage, these demands also promote the lithium-ion battery Research work continues to move forward. From the battery structure and the use of new materials, the new system, the lovely lithium-ion battery researchers have been trying various ways to improve the energy density of lithium-ion batteries.
 
      Structural design
 
      Improve the specific energy of lithium-ion battery from the structure, to improve the positive and negative active substances in the proportion of lithium-ion batteries. Lithium-ion batteries mainly by the positive and negative active substances, diaphragms, copper foil, aluminum foil and shell and structural parts and other components, which can really provide lithium-ion battery capacity only active substances, so improve the active substances in lithium-ion battery The proportion is the most effective way to improve the lithium-ion battery. For example, the recent Tesla in vigorously promote the 21700 battery, is through the use of larger diameter batteries (21mm), increase the height of the battery (70mm) to increase the proportion of active substances, reduce the structural parts and other non-reactive materials, Lithium-ion battery than the energy, reduce the cost per unit watts. In addition, flexible battery is also an effective way to reduce the weight of structural parts, through the use of aluminum plastic film instead of the traditional steel shell, can greatly reduce the structure of lithium-ion batteries in the proportion.
 
      In addition to increasing the diameter of lithium-ion battery, another effective way to improve the specific energy of lithium-ion battery is to reduce the thickness of the diaphragm, the current common PP-PE-PP three-layer composite diaphragm thickness is generally more than 30um to reach the positive and negative pole The thickness of the film about 20%, which also caused a serious waste of space, in order to reduce the space occupied by the diaphragm, the majority of lithium-ion battery manufacturers generally use a thin film with a coating, the thickness of these diaphragms can reach 20um the following, Can ensure the safety of lithium-ion battery under the premise of a significant reduction in the proportion of the volume of the diaphragm to improve the proportion of active substances to improve the specific energy of lithium-ion battery.
      Another way to increase the proportion of active substances is from the battery production process point of view, the first is to increase the proportion of active substances in the electrode. In general, the electrode of lithium ion battery is mainly composed of four parts, active substance, conductive agent, binder and current collector. In order to increase the proportion of active substances, it is necessary to reduce the proportion of other parts. By adopting new conductive agent, Thereby reducing the proportion of the conductive agent and the binder, using a thinner current collector to reduce the proportion of non-active substances. Secondly, it is necessary to increase the coating amount of the positive and negative electrodes, but the problem of increasing the coating amount of the electrode is that when the electrode is too thick, the Li + diffusion kinetics of the electrode is deteriorated, affecting the magnification and circulation of the lithium ion battery Performance, in order to solve this problem Boris Bitsch et al. [1] using the capillary suspension and multi-layer electrode process prepared with gradient porosity of high-performance thick electrode. In the lower layer near the copper foil, Boris Bitsch and so on using a common slurry, making it has a low porosity and good conductivity, and away from the copper foil surface, Boris Bitsch is used capillary suspension slurry, And the addition of 1-octanol, so that its porosity increased significantly to improve the electrode dynamic conditions, so that the porosity of the electrode from bottom to top showed a gradual increase in the characteristics of a significant improvement in the power of the thick electrode Learning conditions to improve the electrochemical performance of the thick electrode, thus achieving the increase in battery weight and volume ratio of energy without reducing the battery cycle performance.
      Another important way to improve the specific energy of lithium-ion batteries is to control the number of electrolytes and reduce the amount of electrolyte can effectively improve the energy density of lithium-ion batteries. Electrolyte in the lithium-ion battery to play a role within the media, positive and negative Li + through the electrolyte to spread, so the electrolyte is theoretically a "non-consumable", as long as a small amount of electrolyte to ensure that Li + The diffusion of the negative electrode on the line, but in fact due to the formation of SEI film during the formation of electrolyte decomposition, and in the cycle of SEI membrane damage and positive oxidation and other causes of electrolyte decomposition, resulting in electrolyte in the actual It is continuous consumption, so the electrolyte inside the battery are generally excessive, which is led to lithium-ion battery than the energy of an important reason, in order to reduce the amount of electrolyte, while ensuring the performance of the battery, we need to electrolysis Liquid solvent system and electrolyte additive system to improve the stability of the electrolyte. In order to improve the stability of the electrolyte in the ternary material NMC battery, Yunxian Qian et al. [2] of the University of Münster, Germany, added a small amount to the traditional EC and EMC (3: 7 weight ratio) FEC additives, found that FEC additives can effectively reduce the decomposition of electrolytes, NMC batteries to improve the efficiency of the first Coulomb, and significantly improve the battery cycle stability.
      2. Selection of positive and negative active substances
 
      As our lithium ion battery energy density requirements continue to improve, the traditional LiCoO2 material has been material to meet the needs of high specific energy lithium-ion battery, in order to further enhance the energy density of lithium-ion batteries, we have two general directions to choose: 1) to improve the working voltage of lithium-ion battery; 2) to improve the capacity of positive and negative materials. First of all, we from the first) to analyze the feasibility of improving the energy density of lithium-ion battery, lithium-ion battery operating voltage depends on the positive and negative electrode material between the voltage difference, the current anode material graphite voltage is very low (about 0.1 (1) high-voltage spinel The working voltage can reach 5.0V, the voltage platform is about 4.7V, the theoretical capacity is 147mAh / g, and the actual capacity can reach 138mAh / g or more. (2) olivine high voltage materials, such as LiMnPO4 and LiCoPO4 materials, etc., which LiMnPO4 material voltage platform can reach 4.1V or so, LiCoPO4 material can reach 4.8V or so. (3) lithium-rich materials, lithium-rich material theoretical capacity of up to 200mAh / g or more, or even 300mAh / g, but in order to play the advantages of high-capacity lithium-rich materials, the need to increase its working voltage, or even about 4.8V The These high-voltage materials in addition to solve their own problems, but also faced with a similar problem: high voltage electrolyte instability problem, the current commercial lithium-ion battery electrolyte is generally based on carbonate-based organic electrolyte, Solvent salt is LiPF6, which also leads to high potential, the electrolyte easily oxidized decomposition, resulting in a serious decline in battery performance, or even security problems. In order to overcome this problem, we can proceed from two aspects, first of all from the electrolyte solvent system, in order to improve the electrochemical stability of the electrolyte window, you can use more stable ionic liquid electrolyte and a new type of electrolyte salt. On the other hand, in order to reduce the oxidation of the high-voltage material to the electrolytic solution, the high-voltage material can be subjected to a surface coating treatment to isolate the electrolyte and the active material. Dongrui Chen et al. [3] used Li3PO4 to cover the lithium-rich layered material. Li3PO4 coating significantly improved the cycling performance of the lithium-rich materials, reduced the dissolution of the transition metal elements, Structure to spinel structure.
 
      Another important aspect of improving the specific energy of lithium-ion batteries is to increase the specific capacity of the positive and negative active substances, which requires the starting from the positive electrode material and the negative electrode material. Cathode materials for our choice of high-capacity cathode materials are the following two categories: 1) ternary materials NCM and NCA; 2) lithium-rich materials. Ternary material is the most mature high-capacity cathode material, and with the increase in Ni content, the specific capacity of ternary materials will be correspondingly increased, such as high-nickel NCM811 material, the specific capacity of up to 200mAh / g or so, The specific capacity of high-nickel NCA material can reach about 190mAh / g, which is much higher than that of LiCoO2. Lithium-rich materials in recent years, the newly developed high-capacity cathode material, the specific capacity can reach 200mAh / g or more, or even 300mAh / g, but the current lithium-rich materials in the market is still relatively rare, the main reason for the following Point: 1. irreversible capacity is high; 2. voltage decay; 3. cycle performance is poor. To improve its performance from the element doping and surface coating, as well as material structure design and other aspects to proceed.
 
      High capacity of the anode material, we mainly have the following options: 1) silicon-based materials; 2) N-doped graphite materials; 3) excessive metal S compounds; 4) metal lithium anode. Silicon negative electrode material Needless to say, which is currently on the market the most mature and reliable high-capacity anode material, crystal Si specific capacity of up to 4200mAh / g or more, but the expansion of large, poor cycle performance, SiOX although the capacity is slightly lower (1500mAh / G), but the cycle performance is excellent, the disadvantage is the first low efficiency. N-doped graphite material is a research hotspot in high-capacity anode materials in recent years. The electronegativity of N atom is about 3.5, and the addition of N element in graphite can significantly improve the specific capacity of graphite anode. Kaifu of Wuhan University Huo et al. [4] prepared an N-doped mesoporous carbon hollow ball material using a stencil method at a current density of 0.131 / g of current density of up to 931 mAh / g and a current density of 0.5 A / g, This is still able to maintain a specific capacity of 485.7mAh / g.
 
      Metal sulfide mainly refers to MoS2, its reversible capacity of up to 1290mAh / g, much higher than the graphite material, but the ion diffusion rate is low, the low electrical conductivity of its performance play, in order to overcome one of the problems, Jie Shao et al [5 The MoS2 nanosheets were grown on N-doped graphite materials, and the materials exhibited good electrochemical performance. The specific capacity was still 915mAh / g at 10mA / g ultra-high current density.
      Metal Li negative specific capacity of up to 3860mAh / g, low potential, good conductivity, is an ideal lithium-ion battery anode material, early because of safety problems were replaced by graphite materials, and in recent years with the Li-S battery And the rapid development of Li-O2 battery, the study of metal Li negative electrode has been gradually deepened, there are a variety of means can inhibit the growth of lithium dendrites, to overcome the metal Li charge and discharge process of volume expansion, improve the safety of metal Li negative, Improve the cycle performance, metal Li negative electrode in the lithium-ion battery application time is ripe, it is estimated that the lithium-ion battery anode replaced by lithium metal, lithium ion battery can increase the energy density to 440Wh / kg or so.