Designing and Demonstrating Advanced Core-Shell Structured Carbon Coated Electrode Materials for High Power Li-Ion Battery Application
Authors: Pratheeksha Parakandy Muzhikara , Rao Tata Narasinga , Anandan Srinivasan 
Published: February 2020

Li-ion batteries (LIBs) have become an integral part of most electronic gadgets, automobiles and (large-scale) grid-power devices due to its intriguing properties such as high energy and power density, no memory effect and excellent cyclic stability. After conquering mobile devices, currently LIBs are under the scanner to gain wide-scale use in commercial (plug-in) hybrid electric vehicles (EVs). These EVs based on LIBs are projected as alternative transport system vis-à-vis the conventional gasoline-based automobiles. However short driving range, slow recharge rate and high cost are the major obstacles and limit their extended use in EVs. Hence, demonstration of large scale LIBs for EV is far more challenging owing to the complex demand of energy density (to deliver 100-200 km per charge) as well as operational safety at viable cost. State-of-the-art lithium-ion cells use layered transition metal oxides, or phosphates or Mn based spinel as cathode and graphitic carbon as anode. Among these, LiFePO4 (LFP) has received much attention as a promising cathode material for high-power lithium-ion batteries used in EVs owing to its abundance, low manufacturing cost, thermal stability, safety, and high theoretical specific capacity. However, low ionic (~ 10-11 to 10-13 cm2 S-1) and intrinsic electronic conductivity (< 10-9 S cm-2) hinder its electro-chemical performance at high rate. Various efforts including (a) doping with supervalent cation, (b) reducing the particle size, and (c) carbon coating have been attempted to overcome these intrinsic defects. Among these efforts, carbon coating has become a widely used solution as it improves the conductivity, prevents metal ion dissolution, avoids direct contact of electrolyte with active material and restricts the crystal growth of electrode material during carbonization process. Though carbon coating is investigated by various methods, most of these methods are struggled by tedious synthesis conditions, non uniform distribution and lack of ordered carbon, which drastically decrease the performance of electrode materials. More importantly, a suitable carbon coating technique which provides collective characteristics of carbon such as (a) low carbon content, (b) thin layer carbon coating, (c) formation of graphitic carbon (d) homogeneous distribution of carbon coating and (e) core-shell structured of carbon coated electrode is yet to be realized.
With this point in mind, a simple, unique and cost effective carbon coating technique to improve the electronic conductivity of electrodes has been discovered. In the present study, we focus on developing carbon coating of lab and large scale (Fig.1A) by dehydration assisted polymerization process, in which usage of dehydrating agent is considered important to form uniform carbon network on LFP. Carbon coating without dehydration agent is also carried out for comparison. The resulting carbon coated electrode material was extensively characterized by various characterization techniques. Structural studies revealed that carbon coating did not alter the crystal structure of LFP and HR-TEM analysis shows the formation of core-shell structure (Fig.1B), i.e., formation of thin layer of carbon (6-8 nm) with less carbon content (3 wt.%) on LiFePO4 particles; ideal for fast lithium ion diffusion during charge/discharge process. The developed carbon coating process led to the formation of sp2 hybridized carbon rich layer by proper catenation of carbon in the presence of dehydration agent, indicating the dominance of graphitic carbon in carbon coated LFP (C-LFP). Characterization studies of carbon coated LFP without dehydrating agent showed that the process was not eminent in yielding required carbon characteristics. The electro-chemical performance of C-LFP was evaluated in half/full cell configuration and benchmark with commercial electrode materials. C-LFP using dehydrating agent exhibit capacity of ~ 85 mA h g-1 at 5C rate with excellent cyclic stability and rate capability, whereas C-LFP without dehydrating agent exhibit a capacity of ~ 43 mA h g-1 at 5 C rate. The specific capacity of C-LFP is 131 mA h g-1 at 1C, whereas the specific capacity of commercially available C-LFP, i.e., UNTPL and TODA is 87 and 143 mA h g-1 respectively (Fig.1C & 1D), demonstrating that developed C-LFP in the present study showing better electro-chemical performance than C-LFP (conventional & UNTPL) and on par with C-LFP (TODA). The excellent electro-chemical performances of C-LFP developed in the present study is attributed to the presence of uniform, thin layer, core-shell structure and high graphitic nature of carbon in carbon coated LFP. Full cell studies of C-LFP with lithium titanate as counter electrode was also carried out and the results are promising. The novel, simple and cost-effective carbon coating process developed in the present study is of potential interest to other low conductive cathode materials and to develop high-power LIBs for EVs application.

Figure 1