The next generation lithium ion batteries (LIBs) will have almost the double the energy density (235 Wh kg^-1) of the currently available LIBs based on lithium transition metal (TM) oxides, olivine cathodes and graphite or silicon anodes for electric vehicles [1]. The batteries should have also reduced cost, improved safety and cycle life. Lithium and manganese rich TM oxide composite cathodes given by the generic formula, xLi₂MnO_3. (1-x)LiMO₂ (Li_1.2Mn_0.55Ni_0.15Co_0.1O_2, LMR-NMC) that has almost double the capacity (~ 300 mAhg^-1) of layered TM oxides, but needs to be electrochemically cycled to a voltage > 4.4V to achieve this high capacity. The high capacity is achieved by activation of Li₂MnO₃ component by charging above 4.4 V forming Li₂O and MnO₂ [2-5]. This contributes to the high irreversible capacities >25% for LMR-NMC electrodes and capacity fade due to significant reduction of electrolytes and electrode conductivity [3]. The capacity fade also associated with Mn dissolution from the host structure upon continuous-s high voltage cycling. Besides, the electrochemical stability of carbonate based electrolyte with LiPF₆ salt will be serious concern that can sustain high voltage cycling (above 4.5V). During high voltage cycling >4.4 V LMR-NMC undergoes a structural transition from layered to the spinel phase resulting significant loss of energy.During cycling, practical energy density of LMR-NMC cathodes are reduced from 1000 Wh kg^-1 during initial cycles to ~750 Wh kg^-1 during 100^th cycle which makes LMR-NMC practically impracticable. The structural transition to the spinel phase is an intrinsic phenomena associated with transition metal ions migrate to the lithium layer during high voltage cycling.

The interfacial instability can be minimized by surface coatings such as metal oxides such as Al₂O₃, ZrO₂, TiO₂ etc., metal phosphate coatings such as AlPO₄, LiFePO₄ etc. blending with another cathode material. In this work, to overcome the issues of interfacial instability (to improve cycle life, C rate performance, irreversible capacity and energy loss, we present a method of LiF coating on to LMR-NMC which stabilizes interface and decreases the voltage fade [4]. In this approach, some of the M-O bonds are replaced by M-F bonds on the surface. The M-F bond is stronger and stabilizes the interphase during cycling. Partially, O^2- is replaced by F^- on the surface of LMR NMC and due to which the average oxidation state of the surface metal ions is slightly decreased which lead to decrease in charge potential thus minimizing the electrolyte decomposition and delivering better electrochemical performance. The fluorine doped cathodes deliver high capacity of ~300 mAh g^-1 at C/10 rate (10-20% greater than the pristine LMR NMC cathodes), have high discharge voltage plateau (> 0.25V ) and low charge voltage plateau (0.2 to 0.4V) compared to pristine LMR NMC cathodes.

Beside fluorine doping, improved interfacial stability and reduce voltage decay can be achieved through both cation and anion dopings. By cation doping Mn or Ni or Co with cations such as Mg, voltage fade has been significantly improved due to structural stabilization. As discussed above, F-substitution stabilizes the surface, helps to reduce charging voltage which is beneficial for LMR-NMC to obtain high capacity at low voltages without electrolyte additives [5]. Further substituting Ni²⁺ with Mg²⁺ helps in minimizing the cation migration as it blocks the tetrahedral void through which movement of cations takes place from transitional metal layer to Li-layer reducing voltage decay. The synergistic effect of both magnesium and fluorine substitution ((Mg-0.02 mole and LiF to LMR-NMC 1:50 mol %) on electrochemical performance of LMR-NMC shows excellent discharge capacity of ~300 mAhg^-1 at C/20 rate whereas pristine LMR-NMC shows the initial capacity around 250 mAhg^-1 in the voltage range between 2.5 and 4.7 V [5]. Mg-F doped LMR-NMC shows lesser Ohmic and charge transfer resistance, cycles very well. The voltage decay which is the major issue of LMR-NMC is minimized in Mg-F doped LMR-NMC compared to pristine and F-LMR -NMC.

By addressing the voltage drop, LMR-NMC could be the new possible cathode material for next generation LIBs.

ACKNOWLEDGMENT

We acknowledge DST-SERB (Grant no. SB/FT/CS-147/2014) for the financial support for this work.

REFERENCES

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5. S. Krishna Kumar, S. Ghosh, S. K. Martha, Ionics 23, 1655–1662 (2017).