Fast-Charging Research On Cathodes And Anodes Of Li-Ion Batteries With High Nickel Layered Oxide/Graphite

Now electric vehicles（EVs）is close to traditional vehicles in terms of cruising range.However,the long charging time still hinders a more widely success of EVs.Fast charging causes many problems,such as reduction in usable energy,accelerated capacity fading,and excessive heat generation.It is significantly to improve fast-charging ability of the materials with high specific capacity and develop a battery technology that supplements abundant energy within a short time.The electrochemical system with high nickel layered oxides/graphite has a huge potential to be improved.However,the intercalation structures themselves are not suitable for fast charging,the high nickel layered oxides also face the influence of multi-stage phase transformations,and most fast-charging researches are based on coin cells,which is greatly different from the real electrochemical environment.Therefore,the failure mechanisms of materials and improvement measures are urgent to be upgraded and put forward,respectively.Works,such as nano-micro-hierarchical structure electrodes with amorphous carbon and graphite and their synergistic mechanism of fast Li-insertion,evolvement mechanism of intergranular cracks in high nickel layered oxides and its retarding strategy were studied.Based on previous works on fast charging electrodes with hard carbon and graphite,a nano-micro-hierarchical structure electrode with carbon black and graphite was constructed by using the characteristics of carbon black,such as isotropy,wider layer spacing and better conductivity.Based on an alkali solution treatment process,high rate charging capacity of graphite was improved,and corresponding activation mechanism was discussed.The evolution processes about single crystal particles of Li Co O₂ and secondary particles of high nickel layered oxides were tracked separately under a fast-charging conditions,two processes occurring synchronously in different dimensions were decoupled,degradation mechanism of cathode materials under fast-charging conditions was comprehensively studied,and several measures to improve cycle stability under fast-charging conditions were proposed.In order to conquer side reactions occurred in high nickel layered oxides during fast-charging process,possibility of Se coating to alleviates the destruction of electrolyte on internal interface was discussed,and efficacy of regulating current density of hybrid electrodes to prolong cycle life under a fast-charging conditions was studied.Finally,based on the optimization of anodes and cathodes,a high-energy-density electrochemical system suitable for fast charging was studied.The following results were obtained:（1）Nano-micron-hierarchical anode constructed by alkali treated graphite and proper amount of carbon black can be fast charged at a high areal capacity of 3.0 m Ah cm^-2.Using KOH to etch graphite at low temperature creates"holes"in basal planes,optimizes edge structures to introduce new lithium ion migration channel and reduce intercalation resistance;moreover,the isotropy and large lattice spacing of carbon black can reduce intercalation resistance and delay Li-plating effectively.（2）Accelerated loss of lithium inventory and decrease of lithium inventory utilization rate in cathodes caused by fast charging jointly lead to a rapid decline of battery capacity under fast charging;unit cells of high nickel layered oxides undergoes significant changes during H2→H3 phase transformation,and accumulated internal stress finally release in a form of intergranular cracks,causing damage to the secondary particles.（3）Fast-charging cycle life of high nickel layered oxides is effectively prolonged by Se-coating treatment and regulating current density during H2→H3 phase transformation.Se diffuses into particle inside along grain boundaries and forms Se-rich protective layers with various valence states,which alleviates the destruction of electrolyte on internal interfaces;the existing of Li Mn₂O₄ in hybrid cathodes shares the current during H2→H3 undertaken by high nickel layered oxides,which delays the cracking of secondary particles;according to the state-of-the-art process of Li-ion batteries,a 20-minute fast charge battery with an energy density of 240 Wh kg^-1 has been successfully achieved.