Regulation Of 3d Transition Metal Elements(Ni,Co&Mn) In Mitigating Voltage Decay Of Li Rich Cathode

With the development of lithium ion batteries from the field of 3C(computers,communications and consumer electronics)to power batteries and large-scale energy storage,the pursuit of high energy density is becoming increasingly urgent.This has also stimulated research enthusiasm for high energy density electrode materials.Among these materials,lithium rich nickel cobalt manganese cathodes(LNCM)have attracted attention due to their high energy density.In recent years,it has been confirmed that the mechanism of high capacity is not only comes from the redox of cations but also the reaction of anion(oxygen).However,the redox couple of oxygen is not stable,which also causes severe voltage decay.In addition,the dissolution of Mn still deteriorates the stability of the cells.Those factors restrict the practical application of these materials.Previous work has shown that increasing the concentration of Ni in LNCM can mitigate voltage decay and maintain the stability of cell.But as a cost,the capacity must be sacrificed.Hence,it’s important to make the balance between voltage decay and capacity.On this basis,LNCM can better meet the"long-lasting" and "low-cost" requirements of electric vehicles and hybrid electric vehicles.(1)LNCM particles with full gradient compounds is prepared by using an ethanol assisted co-precipitation method.The rationally designed procedure involves gradient distributions of transition-metal ions,in which the Ni content increases continuously,Mn content decreases gradually and Co keeps at a low level from the inner to the outside.The employing ethanol during the synthesis adjusts the surface state to further improve the rate performance of the sample.The full gradient cathode not only provides superior cycling capacity,but also improves operating voltage and mitigates voltage decay due to the high Ni/Mn ratio of out layer.The prepared gradient material exhibits high specific capacity of 213.1 mAh g-1 after 100 cycling with voltage decay per cycle about 3.36 mV at 1 C.Under the same test conditions,the control group has a capacity of only 158.4 mAh g-1,and the average voltage drop per cycle is 5.62 mV.Obviously,the gradient sample showed better stability during the cycling.Therefore,we believe this state-of-art full gradient cathode provides an interesting model to promote a further application of LNCM.(2)Tailoring of gradient particle with mitigated voltage decay by the synergistic effect of Li rich and high Ni layered cathodes.To do so,micro sized Li1.20Ni0.13Co0.13Mn0.54O2 particles are mixed with much smaller LiNi0.8Co0.1Mn0.1O2 particles to form deposits onto the larger particles.The concentration gradient of Ni is then achieved as the Ni ions in LiNi0.8Co0.1Mn0.1O2 penetrate into Li1.20Ni0.13Co0.13Mn0.54O2 during a calcination post treatment.It is found that not only the Li2MnO3 component in the gradient particles is limited but also few spinel domains are formed during synthesis,which would play positive roles in the stability of the material.The gradient cathode(10%NCM811)induced by 10 wt%LiNi0.8Co0.1Mn0.1O2 exhibits an average particle size of 15 μm and the discharge capacity at 0.1 C is 264 mAh g-1.In the 0.5 C charge-discharge cycling,the average voltage drop per cycle is only 1.53 mV.And the capacity retention can reach 90.6%,which is higher than that of control group with 83.5%.Besides,10%NCM811 gradient sample showed superior thermal stability.The weight loss of the delithiated 10%NCM811 sample at high temperature is 10.1%,which is smaller than that of the control group(12.1%).The thermal analysis indicates that the oxygen loss of the gradient sample is alleviated.In summary,this systematic study explores a material model combining Li rich and high Ni layered cathodes that is shown to be effective in making balance between mitigated voltage decay and high energy density.(3)Investigating of the effect of Ni concentration on the stability of LNCM.By studying the structure,electrochemical performance and thermodynamic stability of materials with different Ni contents,the role of Ni in LNCM is investigated in detail.It is found that appropriately increasing the Ni content can suppress the voltage decay of LNCM,however,when the Ni content in the material is too high,the system is not stable anymore.Among those samples,0.4Li2MnO3·0.6LiNi0.6Co02Mn0.2O2(LNCM622)exhibits the best performance.The average voltage drop per cycle at 0.5 C is 1.06 mV,which is less than those of the highest Mn(lowest Ni)content cathode 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2(LNCM111,1.82 mV)and the highest Ni content cathode 0.4Li2MnO3·0.6LiNi0.8Co0.1Mn0.1O2(LNCM811,1.33 mV).Besides,the energy density of the LNCM622(755.7 Wh kg-1)is higher than those of LNCM111(634.0 Wh kg-1)and LNCM811(466.5 Wh kg-1)after 100 cycles.The results of thermodynamic analysis further reveal that if we increase the Ni or Mn amount to a very high level in the LNCM,the oxygen loss associated with voltage decay would become serious.Therefore,the role of Ni in mitigating voltage decay is only effective at appropriate transition metal ratios.An excessively concentration of Mn will cause a rapid transformation of the layered structure to a spinel structure,leading to voltage decay.And high concentration of Ni is also thermodynamic metastable.Only under the proper Ni/Mn ratio can a stable Li rich cathode be obtained,eg LNCM622 in this work.Further in-situ XRD also proves the stability of this component.(4)Exploring the interactions between electrodes and electrolyte,as well as the impacts of the cathode’s characteristics on the Li metal anode.According to the results of ICP-OES,it can be found that along with the increased Ni amount in the Li rich materials,the Ni dissolution in electrolyte is serious.This result further confirms that the unstable factor caused by high concentration of Ni ions should be considered in the design of cathode materials.In addition,the thickness of the cathode-electrolyte interface layer(CEI)and the heat release of the electrode are highly depended on the proportion of the 3d transition metal in the cathode.The thicknesses of CEI for LNCM111,LNCM622,and LNCM811 are 0.68 nm,0.53 nm,and 1.65 nm,respectively.The CEI for LNCM622 electrodes is thinnest,which is more conducive to Li+transmission.The DSC results still indicate the moderate heat release and reaction temperature for LNCM622.In addition,Li deposition/dissolution behaviors and kinetics characters on Li anode have been investigated in this work.It is clear to find that the use of LNCM622 as cathode leads to a larger size of Li deposits and a lower over potential on Li anode,which may alleviate the the formation of "lithium dendrites" and "dead lithium".We hope to find a suitable 3d transition metal ratio(eg LNCM622)in Li rich cathode,which can not only limit voltage decay,but also show better compatibility with electrolyte and Li metal anode.