Bio-Derived Lithium-Ion Battery Anode Material for Fast Charging and Long-Cycle Life

Abstract

Currently graphite is the most commercially used anode material for lithium-ion batteries (LIB), however its limited theoretical capacity of 372 mAhg-1 and slow kinetics cannot satisfy the requirements for EVs. Hence, alternative high energy density anode materials with fast kinetics are the need of the hour. The most recent development involves Mitsubishi Outlander PHEV which takes around 40 minutes for empty to full charging. Extreme fast charging, with a goal of 15 minutes fast charging, is poised to accelerate mass market production of electric vehicles. Extensive research is now directed towards carbonaceous materials that can fulfill the requirements for EV application. Key strategies involved in improving the rate capability of carbonaceous materials are (a) heteroatom doping, especially nitrogen doping and (b) increasing the d-spacing for accessing more active sites. Using conventional methods of heteroatom doping like pyrolysis of graphene oxide with urea or melamine as nitrogen source led to inclusion of up to 12 at% of nitrogen in general. Inclusion of nitrogen showed improvement in rate capability and capacity. Further search to increase nitrogen content led to the application of graphitic carbon nitride with nearly 55 at% nitrogen. Nevertheless, due to less electronic conductivity and change of crystallinity upon lithiation curbed its application in lithium-ion battery. So, currently our objective is to use nitrogen rich polymer as a single source of nitrogen and carbon to prepare heavily nitrogen doped carbon with nearly 20 at% of nitrogen with increased d-spacing and understand its effect on Li ion storage. Poly (2, 5-benzimidazole) (PBI) was synthesized by homo-polycondensation method using a bio derivable starting material 3, 4-diaminobenzoic, as earlier reported by Kaneko et.al1. The as prepared PBI was ground into fine powder before pyrolyzing at 800 ℃ in nitrogen atmosphere. Obtained carbon material was washed with HCl and water repeatedly and dried at 80 ℃ under vacuum before grinding into fine powder using mortar and pestle. Obtained material was systematically characterized before preparation of anodes for Li ion battery application. The SEM-EDX data revealed the content of nitrogen was as high as 18 at%. Using XPS studies, different types of nitrogen moieties were identified. Deconvolution of N1s peak revealed the presence of nitrogen in four different forms, graphitic nitrogen (10.3%), pyridinic nitrogen (6%), pyrrolic nitrogen (0.6%) and oxidized nitrogen (0.6%). The presence of pyridinic and graphitic nitrogen improve the electrical conductivity. The presence of high pyridinic nitrogen results in improved electrochemical activity. The contribution of pyridinic nitrogen in lithium storage is known to be maximum. XRD technique was utilized to understand the d-spacing of the material. Reflection corresponding to C (002) was used to calculate d-spacing. The results revealed that the d-spacing was found to be 3.5 Å which is nearly 0.2 Å higher than conventional graphite. The increased d-spacing can promote in faster diffusion of Li ion during intercalation and de-intercalation. TEM micrographs revealed the presence of macropores and mesopores. Using Cyclic Voltammograms at different scan rates and Randles Sevcik equation, the diffusion coefficient was calculated. Diffusion coefficient of PY PBI 800 was found to be several times higher than the conventional graphite anode, attributed to the increased d-spacing along with porous nature. Upon galvanostatic charge-discharge studies at different current rates, anode showed reversible capacity with nearly 99% coulombic efficiency upto 5C (1.8 Ag-1) rate. Further, long cycling studies were performed for >1000 cycles at 0.4, 0.8 and 1.8Ag-1 rates. Results indicated that PY PBI 800 can deliver highest de-lithiation capacity of ̴ 260 mAhg-1 at specific capacity of 0.4 Ag-1 rate (Fig) with nearly 88% capacity retention after 1000 cycles. At higher current rate of 0.8Ag-1, and 1.8 Ag-1 it showed highest discharge capacity of 165 mAhg-1 and 135 mAhg-1 with 90% and 75% capacity retention after 1000 cycles respectively. At 5C current rate, the battery can be charged and discharged in just 9 mins. Further, full cell was studied with LiCoO2 as cathode and PY PBI 800 as anode. Full cell studies also revealed the promising nature of PY PBI 800. Thus, carbonization is found to be a single method to enable high N-doping and increased d-spacing. The battery’s performance of charge discharge in just 9 min at 5C, with good capacity retention of nearly 75% after 1000 cycles demonstrated its promising potential for EV application.