Revealing Rate Limitations in Nanocrystalline Li₄Ti₅O₁₂ Anodes for High‐Power Lithium Ion Batteries

Abstract

because of their high energy density, long cycle-life, the lack of a memory effect, and relative environmental benignity. These qualities make LIB successfully employed in portable electric devices (e.g., laptops, cell phones, and digital camera), transportation, local grid energy storage, and aerospace. [5][6][7][8][9] Graphite is commonly used as anode material in state-of-the-art LIBs; however, its lower lithium (Li) insertion potential (below 0.2 V Li + /Li) may result in the deposition of metallic Li and thereby the formation of Li dendrites on electrode surface when the batteries are over-charged or cycled at low temperatures, which lead to a signifi cant safety hazard for large scale applications. [10][11][12] Besides, the poor ratecapability and unsatisfi ed cycling stability that results from the Li ion diffusion orientation and solvent cointercalation characteristics in layered crystal structure still makes graphite unable to meet the increasing rate performance demand for next generation of high power LIBs. [ 3,13,14 ] Therefore, there is an intense need to explore alternative materials that can effectively overcome the issues arising from using graphite.Among the alternatives, spinel lithium titanium oxide (Li 4 Ti 5 O 12 : 175 mAhg −1 ) has attracted much attention as a new anode candidate owing to its intrinsic characteristics of the higher Li insertion potential (1.55 V) versus Li + /Li and the "zero-strain" characteristic during Li insertion/extraction as well as high thermal stability in both charged and discharged states. [ 4,[15][16][17] The high and stable lithiation potential can avoid not only the dendritic Li growth, but also minimize the formation of solid-electrolyte interphase (SEI) fi lm, leading to greatly improved safety and high initial coulombic effi ciency of the electrode. [ 18,19 ] Despite of these advantages, the calculation of density of states (DOS) of Li 4 Ti 5 O 12 gives a band gap energy of ≈2 eV, [20][21][22][23] indicating its insulating characteristic, which endow Li 4 Ti 5 O 12 with a very low electronic conductivity (<10 −13 S cm −1 ). [ 24 ] This has been viewed as a major obstacle for increasing rate performance of Li 4 Ti 5 O 12 . [24][25][26][27][28][29][30][31][32][33] The approaches to enhance the electronic conductivity include donor-doping the lattice of Li 4 Ti 5 O 12 with aliovalent cation (e.g., Mg 2+ , Al 3+ , Nb 5+ , Ta 5+ ) at Li + or Ti 4+ sites to get mixed Ti 4+ /Ti 3+ valence [24][25][26][27] and introducing directly conductive second phase (e.g., Ag, Cu, C, carbon nanotubes (CNTs), graphene) to form composite. [28][29][30][31][32][33] Mg substitution Li 4 Ti 5 O 12 is a promising anode material for lithium ion batteries due to its high safety, excellent cycling stability, environmental friendliness, and low cost. Strategies of incorporation with a conductive component (such as carbon) and constructing nano-structure are frequently adopted to improve the rate-

One‐Step Synthesis of Highly Oxygen‐Deficient Lithium Titanate Oxide with Conformal Amorphous Carbon Coating as Anode Material for Lithium Ion Batteries

“…The defective‐LTO displayed a pair of sharp and intense peaks at about 1.51 and 1.71 V, while the pristine‐LTO has a pair of peaks at about 1.43 and 1.77 V, with much weaker peak current and peak symmetry. This reveals higher electrochemical activity and faster reaction kinetics for the defective‐LTO . The electrochemical window, the potential difference between the cathodic and anodic peaks, was much smaller for the defective‐LTO compared to that for the pristine‐LTO by 0.17 mV.…”

“…The compounding effect of both surface modifications, namely, the high level of oxygen vacancy and conformal carbon coating, changed the chemical composition at the interface which lead to superior interfacial electronic conductivity and charge transfer for LTO. This clear set of studies sheds light on a very controversial discussion recently regarding which step or steps are rate limiting for improving the performance of LTO anode . The influence of R ct on the defective‐LTO as an anode can be even highlighted when correlated with the rate capability in the following electrochemical testing.…”

“…With amorphous carbon coating, Chen and co‐workers demonstrated lower Li ion (Li + ) interfacial transfer resistance which dramatically enhanced the battery performance . Altogether achieving a “double” effect, Wang et al introduced nanosized LTO with surface modifications of Ti (III) and carbon with improved surface conductivity and restricted particle growth due to the carbonization of polyaniline (PANI); however, they were not able to maximize the effects of oxygen vacancies by generating a low level; and their nonuniform carbon layers can possibly impede Li + transport, especially when graphitized . Finally, the importance of these modifications in relation to the Li + interfacial charge‐transfer resistance was not highlighted in the aforementioned studies.…”

Impact of Carbon Nanostructures as Additives with Spinel Li4Ti5O12/LiMn2O4 Electrodes for Lithium Ion Battery Technology

“…Nevertheless, its poor electronic conductivity is still a challenge . Approaches to improve LTO conductivity through techniques like in‐situ and ex‐situ carbon coating, particle size reduction, doping etc are extensively being explored ,,,. On the other hand, spinel LMO cathode has also engrossed intense attention due to its high voltage, three dimensional diffusion pathways, non‐toxicity and abundance .…”

“…[10] Approaches to improve LTO conductivity through techniques like in-situ and ex-situ carbon coating, particle size reduction, doping etc are extensively being explored. [6,7,11,12] On the other hand, spinel LMO cathode has also engrossed intense attention due to its high voltage, three dimensional diffusion pathways, non-toxicity and abundance. [13] Yet the dissolution of Mn 2 + leading to rigorous capacity loss is a key hindrance.…”