Li4Ti5O12 Surface Research Articles

Temperature Dependence of Electrochemical Degradation in LiNi1/3Mn1/3Co1/3O2/Li4Ti5O12 Cells

Aging mechanisms in lithium‐ion batteries are dependent on the operational temperature, but the detailed mechanisms on what processes take place at what temperatures are still lacking. The electrochemical performance and capacity fading of the common cell chemistry LiNi1/3Mn1/3Co1/3O2 (NMC)/Li4Ti5O12 (LTO) pouch cells are studied at temperatures 10, 30, and 55 °C. The full cells are cycled with a moderate upper cutoff potential of 4.3 V versus Li+/Li. The electrode interfaces are characterized postmortem using photoelectron spectroscopy techniques (soft X‐ray photoelectron spectroscopy [SOXPES], hard X‐ray photoelectron spectroscopy [HAXPES], and X‐ray absorption near edge structure [XANES]). Stable cycling at 30 °C is explained by electrolyte reduction forming a stabilizing interphase, thereby preventing further degradation. This initial reaction, between LTO and the electrolyte, seems to be beneficial for the NMC–LTO full cell. At 55 °C, continuous electrolyte reduction and capacity fading are observed. It leads to the formation of a thicker surface layer of organic species on the LTO surface than at 30 °C, contributing to an increased voltage hysteresis. At 10 °C, large cell‐resistances are observed, caused by poor electrolyte conductivity in combination with a relatively thicker and LixPFy‐rich surface layer on LTO, which limit the capacity.

N-functionalized graphene quantum dots: Charge transporting layer for high-rate and durable Li4Ti5O12-based Li-ion battery

Spinel Li4Ti5O12 can replace carbon in Li-ion battery anodes due to its high voltage, preventing decomposition of the electrolyte and formation of Li metal dendrites. However, Li4Ti5O12 has a low electronic conductivity and Li-ion diffusion coefficient, limiting its charge/discharge properties at high rate capacities, and also suffers from gassing during cycling. Here, we used N-functionalized graphene quantum dots interfacial layer, which (1) protects Li4Ti5O12 from ambient degradation, (2) forms a thin and smooth solid-electrolyte interphase layer on the Li4Ti5O12 surface, (3) acts as a charge transfer layer, (4) protects the Li4Ti5O12 electrode from reactions with the electrolyte, and (5) suppresses gassing during cycling. Consequently, the Li-ion diffusion coefficient increased by ∼19%. The effectiveness of the N-functionalized graphene quantum dots is manifested in the specific capacity of 161 mAh/g at 50C, which is improved by ∼23% compared to pure Li4Ti5O12 electrode and maintained for over 500 cycles. Unlike graphene, N-functionalized graphene quantum dots themselves work as a stable charge transporting and protecting layer. Our strategy successfully obtained a good cycling performance and long cycling life of Li4Ti5O12 at high C-rates.

Doping and surface modification enhance the applicability of Li4Ti5O12 microspheres as high-rate anode materials for lithium ion batteries

In this study we prepared Mg2+ and Cr3+ co-doped and phosphidated Li4Ti5O12 (LTO) microspheres using a spray-drying method. Doping with both Mg2+ and Cr3+ improved the electronic conduction of LTO, due to partial reduction of Ti4+ (from Ti4+ to Ti3+) and narrowing of the band gap, respectively. Moreover, the phosphidation process formed a uniform conductive glass layer on the LTO surface, leading to higher ionic conductivity. As a result, the modified LTO exhibited improved rate capability and cyclic stability, compared with those of the pristine LTO. In particular, the Mg2+ and Cr3+ co-doped and phosphidated LTO exhibited superior performance as a result of the combined effects of ion-doping and interfacial modification. Finally, the capacity improved significantly, especially at high C rates [e.g., 127.72 mA h g–1 at 20 C—77.6% of the value recorded at 0.1 C (164.62 mA h g–1)], due to the dual effects of the enhanced electric and ionic conductivities.

Overcharge durability of Li4Ti5O12 based lithium-ion batteries at low temperature

Abstract The decrease in capacity at low temperature limits further application of lithium-ion batteries. This paper introduces an approach to compensating the capacity loss of Li4Ti5O12 (LTO) based lithium-ion batteries at low temperature by increasing the charging cut-off voltage. The impact of this approach on battery durability is examined afterwards, and a “tipping point” voltage for degradation is observed. Cut-off voltages over the tipping point (3.2 V) cause severe degradation, while those beneath the tipping point do not. The degradation mechanisms are studied with incremental capacity (IC) curve analysis, a prognostic and mechanistic (P&M) model and Scanning Electron Microscope/X-Ray Energy Dispersive Spectrometer (SEM/EDS) tests. Analysis show that the potential of the LTO anode first drops below 1 V when the battery is overcharged to 3.2 V. The electrolyte exceeds the reduction potential and decomposes on the LTO surface, forming Solid Electrolyte Interface (SEI) film with gas generation. The anode potential then gradually rises to above 1 V as we continue to charge the battery. However, the cathode potential exceeds 4.6 V, resulting in electrolyte oxidation and active material loss in the cathode. Overall, enhancing the cut-off voltage from 2.7 V to 3.2 V is effective to enlarge the charging capacity (9.5%) of the LTO batteries at −20℃ with slight degradation.

Influence of carbon coating and PANI modification on the electrochemical performance of Li4Ti5O12

The Li4Ti5O12/polyaniline and Li4Ti5O12/С/polyaniline composite materials with different lithium titanate/aniline ratio of 8:1, 4:1, 2:1, 1:1, 1:2, and 1:4 were synthesized by in situ polyaniline (PANI) polymerization. The materials were characterized by high-resolution TEM, X-ray powder diffraction, IR spectroscopy, and TGA. It was shown that polyaniline is uniformly distributed over the surface of Li4Ti5O12 or Li4Ti5O12/С composite. For the sample with the minimum polyaniline content (Li4Ti5O12/С/PANI (8:1)), the PANI layer thickness is 2–3 nm and contains polyaniline in the most conductive (emeraldine) form. The lithium titanate modification with polyaniline results in sharp increase in the electronic conductivity of materials from ~ 10−11 Ohm-1 cm−1 for Li4Ti5O12 to 10−4 Ohm-1 cm−1 for Li4Ti5O12/C/PANI. With increasing polyaniline content, the electrochemical capacity decreases for all samples, including carbon-coated and uncoated ones. The highest discharge capacity at current density < 800 mA g−1 is attained for the Li4Ti5O12/С/PANI (8:1) composite.

A facile electrochemical modification route in molten salt for Ti3+ self-doped spinel lithium titanate

Ti3+ self-doped spinel Li4Ti5O12 (LTO) particles were synthesized through a facile electrochemical modification route in NaCl-KCl molten salt electrolysis, and the formation mechanism is explored. Alternating Ti3+ self-doping significantly improves the electrochemical performance of the spinel Li4Ti5O12 (LTO) particle, especially at high charge/discharge rates. According to the report, LTO, once introduced with oxygen vacancies or Ti3+, will decrease the band gap about to 1.54 eV, which can dramatically improve the inherent electronic conductivity. Through the molten salt electrochemical modification technique, the Ti3+ species will be doped onto the surface of LTO, which was confirmed through transmission electron microscopy with EPR (namely electron spin resonance) spectra, X-ray photoelectron spectroscopy, X-ray analysis. As anodes in lithium-ion batteries (LIBs), the spinel Li4Ti5O12 (LTO) particle electrode with self-doped Ti3+ can deliver stable discharge capacities of 168, 152, 131, 120, 102, 93 and 78 mAh g−1 at different rates of 0.5, 1, 5, 10, 15 and 20C, respectively. Meanwhile, like pure spinel LTO, they also carry strong cycling stability and demonstrate the capacity retention of 92.0%, though after 900 cycles under 5C. Our results indicate that pure-phase LTO is prepared by a molten salt method, and it is then electrochemically modified by a constant current to obtain conductive Ti3+. Self-doped spinel Li4Ti5O12 particles are a good alternative to facilitate the transfer of electrons, especially under high-rate conditions, because it shows good electronic and ionic conductivities. In addition, the molten salt synthesis and the electrochemical modification steps of this method are conducted in the same reactor with a simple process, strong operability and environmental friendly process.

Effects of carbon coating from sucrose and PVDF on electrochemical performance of Li4Ti5O12/C composites in different potential ranges

The carbon coated nanoflower-like Li4Ti5O12/C composites were prepared via hydrothermal method followed by surface modification using sucrose or polyvinylidene fluoride (PVDF) as carbon sources. X-ray diffraction, SEM, TEM, Raman spectroscopy, TGA, and the electrochemical measurements were used for the materials characterization. Such modification leads to the formation of a high-conductive carbon coating. In the case of polyvinylidene fluoride use, fluorination of Li4Ti5O12 surface takes place also. As a result, electrochemical performance of the obtained composites is improved. In the potential range of 1–3 V, Li4Ti5O12, Li4Ti5O12/CPVDF, and Li4Ti5O12/Csucrose exhibit, respectively, the discharge capacities of 142.5, 154.3, and 170.4 mAh/g at a current of 20 mA/g and 57.2, 82.1, and 89.3mAh/g at a current of 3200 mA/g. When cycled in a potential range of 0.01–3 V, the discharge capacity of Li4Ti5O12/CPVDF increases up to 252 mAh/g at 20 mA/g.

Enhanced electrochemical performance of Li4Ti5O12 through in-situ coating 70Li2S-30P2S5 solid electrolyte for all-solid-state lithium batteries

High energy and power densities are the urgent task for all-solid-state lithium batteries (ASSLIBs). Intimate contact at electrode/solid electrolyte interface and high ionic conductivity of solid electrolyte are crucial to realize high-performance all-solid-state lithium batteries. Here, we provide a liquid-phase approach to in-situ coating 70Li2S-30P2S5 (LPS) solid electrolyte onto the surface of Li4Ti5O12 (LTO) nanoparticles from mineral spirit. The cells employing LTO@LPS material in combination with the LPS solid electrolyte, vapor grown carbon fibers (VGCFs) and polybutadiene (PB) as cathode and lithium metal as anode exhibit excellent rate capacity and cycling stability, showing reversible capacity of 110 mAh g−1 under a load for 0.5 mg cm−2 (with respect to LTO) at 1 C (1 C = 175 mA g−1) and 80 °C after 300 cycles, it even produces 118 mA h g-1 at 0.1 C under a high load for 4.25 mg cm−2.

Effect of Electrolyte Additives on Li4Ti5O12 Cycling Performance and Gas Evolution

Batteries consisting of Li4Ti5O12 (LTO) anodes do not require the formation of a solid electrolyte interface to deliver robust high-rate performance at room temperature, however performance suffers at elevated temperatures due to gas evolution. Research has linked gas evolution to the instability of the electrolyte on the surface of charged LTO at elevated temperatures. If this is the case, a passivation layer, which prevents the electrolyte from coming into contact with the charged surface of LTO, should inhibit gas evolution. Several classes of electrolyte additives have been investigated in Li4Ti5O12/LiMn2O4 coin cells and pouch cells. ATR-IR and X-ray photoelectron spectroscopy has been used to gain an understanding of the surface films formed with different additives while in-situ gas measurements based on Archimedes’ principle and gas chromatography have given insight into how the implementation of these additives affects gassing. The results from this study enable the selective design of surface films for LTO anodes, which reduces gassing at elevated temperatures without sacrificing performance.

Study of the Hydrate-Melt/Li4Ti5O12 Interphase by Scanning Electron Microscopy-Based Spectroscopy.

To develop safe and low-cost Li-ion batteries (LIBs), recently, an aqueous-based electrolyte so-called "hydrate-melt" (HDM) electrolyte is proposed. Li4Ti5O12 is a promising negative electrode material for a LIB with such a HDM electrolyte because of its unexpected reversible Li insertion and extraction properties without usually inevitable water reduction. The solid-electrolyte interphase formation is one of the reasons for this stable reaction, although a detailed analysis is not yet performed. Here, a Li4Ti5O12 electrode surface reacted in a HDM electrolyte is investigated by scanning electron microscopy-based analysis. Surface reaction products are clearly observed on the Li4Ti5O12 surface after the Li insertion reaction in a HDM electrolyte. Energy-dispersive X-ray spectroscopy and Auger electron spectroscopy indicated that the products do not contain any components originated from Li salts, whereas anion-derived passivation films seem to cover a bare surface below the products. Further, the surface products are identified as Li2O by the feature of Li-K-edge reflection electron energy-loss spectrum. The Li2O formation would be one of the key issues for stable Li insertion and extraction of a Li4Ti5O12 electrode in a HDM electrolyte.

MoO2 nanoparticle-modified Li4Ti5O12 as high rate anode material for lithium ion batteries

Abstract MoO2 nanoparticle-modified Li4Ti5O12 materials were prepared via a facile solution-based method. X-ray diffraction patterns (XRD) and high resolution transmission electron microscopy (HRTEM) analyses revealed that the MoO2 nanoparticles were anchored on the surface of spinel Li4Ti5O12, and energy dispersive X-ray spectroscopy (EDX) mapping indicated the MoO2 nanoparticles distributed uniformly. Compared with the pristine Li4Ti5O12, the appropriate amount of MoO2 on the surface of the Li4Ti5O12 could not only effectively reduce the electrochemical polarization of Li4Ti5O12, but also contribute to specific capacity for lithium storage. The 3% MoO2 modified Li4Ti5O12 exhibited excellent rate capability of 116 mA h g−1 at 10 C, and superior cycling performance of 124 mA h g−1 after 450 cycles at 5 C. Since the MoO2 modification process was easy to handle and the source of Mo has affordable costs, MoO2-modified Li4Ti5O12 would be promising for industrial application.

A Facile Surface Reconstruction Mechanism toward Better Electrochemical Performance of Li4Ti5O12 in Lithium-Ion Battery.

Through a facile sodium sulfide (Na2S)‐assisted hydrothermal treatment, clean and nondefective surfaces are constructed on micrometer‐sized Li4Ti5O12 particles. The remarkable improvement of surface quality shows a higher first cycle Coulombic efficiency (≈95%), a significantly enhanced cycling performance, and a better rate capability in electrochemical measurements. A combined study of Raman spectroscopy and inductive coupled plasma emission spectroscopy reveals that the evolution of Li4Ti5O12 surface in a water‐based hydrothermal environment is a hydrolysis–recrystallization process, which can introduce a new phase of anatase‐TiO2. While, with a small amount of Na2S (0.004 mol L−1 at least), the spinel‐Li4Ti5O12 phase is maintained without a second phase. During this process, the alkaline environment created by Na2S and the surface adsorption of the sulfur‐containing group (HS− or S2−) can suppress the recrystallization of anatase‐TiO2 and renew the particle surfaces. This finding gives a better understanding of the surface–property relationship on Li4Ti5O12 and guidance on preparation and modification of electrode material other than coating or doping.

New synthesis route to decorate Li4Ti5O12 grains with GO flakes

Lithium titanium oxide (Li4Ti5O12) particles were surface-decorated with 1–5% wt. of graphene oxide (GO) using new low temperature method (LTM). All the synthesized materials have been characterized by a number of methods: X-ray powder diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The study includes the influence of the graphene oxide flakes decoration on lithium titanium oxide crystal structure. Raman analysis demonstrates a typical spinel spectra of Li4Ti5O12 (LTO) and relatively sharp D and G lines of GO structure. The XPS measurements shows the most intense peaks at the binding energy of 284.5 eV, which corresponds to CC/CC in aromatic rings of GO located on the surface of Li4Ti5O12. The peaks observed at 285.8, 286.9 and 288.3 eV belong to CO in hydroxyl and epoxy groups, carbonyl functional groups, and OCO in carboxyl groups, respectively. The correlation between the structural and morphological analysis of spinel Li4Ti5O12 structure decorated with graphene oxide (GO) flakes was examined for the first time in this work.

The Effects of Electrolyte Additives on Li4Ti5O12 and How They Impact Gassing

Batteries consisting of Li4Ti5O12 (LTO) anodes do not require the formation of a solid electrolyte interface to deliver robust high-rate performance at room temperature, however performance suffers at elevated temperatures due to gassing. Research has linked this gassing to the instability of the electrolyte on the surface of charged LTO at elevated temperatures. If this is the case a passivation layer that prevents the electrolyte from coming into contact with the charged surface of LTO should hinder gassing. Several classes of electrolyte additives have been investigated in Li4Ti5O12/LiMn2O4 coin cells and pouch cells. ATR-FTIR and X-ray photoelectron spectroscopy has been used to gain an understanding of the surface films formed with different additives while in-situ gas measurements based on Archimedes’ principle and gas chromatography have given insight into how the implementation of these additives affects gassing. The results from this study enable the selective design of surface films for LTO anodes, which impedes gassing at elevated temperatures without sacrificing performance.

Performances and Gassing Behavior of Mixed Ti/Nb Oxides As High Potential Li-Ion Anodes

TiNb2O7 and Ti2Nb10O29 were first reported by Roth [1] and Wadsley [2] in the 1950s, however it is only in 1983 that Cava [3] showed the possibility of chemical insertion of Li+ in their sheared structures. Han and Goodenough [4,5] followed 28 years later with the first report on the electrochemical activity of sol–gel TiNb2O7 showing promising capacities and rate performances along with its full cell performances with a LiNi0.5Mn1.5O4 cathode in the coin cell format. These interesting performances make it a serious contender to the replacement of Li4Ti5O12 as anode material. Between 2013 and 2016, a large number of studies were dedicated to optimize TiNb2O7 morphology and composition to increase its electrochemical performances [6-9]. On the other hand, the electrochemical performances of Ti2Nb10O29 have only been reported after the solid state reaction [10]. Amongst those reports, only few have evaluated the electrochemical performances of those two oxides in Li-ion configuration and always at a coin cell level. Evaluation in a larger size pouch cell format is a pre-requisite prior to industrial consideration. The common swelling of Li4Ti5O12 based soft shell full cells has been one of the (safety) limitations preventing further industrial development. The first mention of package swelling was reported for Li4Ti5O12/LiMn2O4 pouch cells by Du Pasquier in 2003 [11]. This issue has only been further investigated more recently by Belharouak et al. [12], Wu et al. [13]. These reports assigned the gassing phenomenon to the electrolysis of water traces, contained in the electrolyte or absorbed on the electrodes, for which Li4Ti5O12 can act as a catalyst. The reduction of water contributes to the release of high concentrations of H2, representing 80 wt% of the generated gases. Further reaction between Ti4+ and carbonate solvents leads to electrolyte decomposition via the generation of CO, CO2 and C1–C3 hydrocarbons, and formation of an SEI layer at the surface of Li4Ti5O12. While further enhancement of electrochemical performances of TiNb2O7 and Ti2Nb10O29 is still a possibility as will be demonstrated in this presentation, the probability of the gassing phenomenon of these two oxides in full cells should be monitored. We will present the development of a synthesis route for both TiNb2O7 and Ti2Nb10O29. The combination of a solvothermal treatment to a ball milling step was designed to enhance the electrochemical performances of the two oxides by (1) increasing the accessible capacity at high cycling rates and (2) limiting the effect of capacity fading over time. In the second part, full cell cycling and gassing properties of as-prepared TiNb2O7 will be reported and compared to the ones of Li4Ti5O12 with LiMn1.5Ni0.5O4 and Li(Ni1/3Mn1/3Co1/3)O2 cathodes. [1] R. Roth and L. Coughanour, J. Res. Natl. Bur. Stand., 1955, 55, 209. [2]A. D. Wadsley, Acta Crystallogr., 1961, 14, 664–670. [3] R. J. Cava, D. W. Murphy and S. M. Zahurak, J. Electrochem.Soc., 1983, 130, 2345–2351. [4] J.-T. Han, Y.-H. Huang and J. B. Goodenough, Chem. Mater., 2011, 23, 2027–2029. [5] J.-T. Han and J. B. Goodenough, Chem. Mater., 2011, 23, 3404–3407. [6] K. Tang, X. Mu, P. A. van Aken, Y. Yu and J. Maier, Adv.Energy Mater., 2013, 3, 49–53. [7] S. Jayaraman, V. Aravindan, P. S. Kumar, W. C. Ling,S. Ramakrishna and S. Madhavi, ACS Appl. Mater.Interfaces, 2014, 6, 8660–8666. [8] B. Guo, X. Yu, X.-G. Sun, M. Chi, Z.-A. Qiao, J. Liu, Y.-S. Hu,X.-Q. Yang, J. B. Goodenough and S. Dai, Energy Environ. Sci.,2014, 7, 2220–2226. [9] H. Park, H. B. Wu, T. Song, X. W. Lou and U. Paik, Adv. Energy Mater., 2015, 5, 1401945–1401951. [10] X. Wu, J. Miao, W. Han, Y.-S. Hu, D. Chen, J.-S. Lee, J. Kim and L. Chen, Electrochem. Commun., 2012, 25, 39–42. [11] A. D. Pasquier, I. Plitz, S. Menocal and G. Amatucci, J. Power Sources, 2003, 115, 171–178. [12] I. Belharouak, G. M. Koenig, T. Tan, H. Yumoto, N. Ota and K. Amine, J. Electrochem. Soc., 2012, 159, A1165–A1170. [13] K. Wu, J. Yang, Y. Zhang, C. Wang and D. Wang, J. Appl. Electrochem., 2012, 42, 989–995. Figure 1

Nanoscale Xpeem Spectroscopy Reveals the Origin and Nature of the Lto Electrode Surface Reactivity

A stable electrode/electrolyte interface is an essential requirement to ensuring safe operation and long-life span of Li-ion batteries. The challenge of assessing such stability relates to the difficulty of determining the exact thermodynamic stability window of the electrolytes. Indeed, the strong mismatch between the theoretical and experimental values and their dependence on the investigated electrode remains a major hurdle in the field. Surprisingly, the presence onf a surface layer, as a result of electrolyte decomposition, has been reported on Li4Ti5O12 (LTO) electrodes operating at a voltage plateau where the electrolyte components are expected to remain stable. Despite the considerable work aimed at elucidating the origin, thickness and homogeneities of such surface layer, there is still a lack of knowledge on the reaction mechanism behind its formation. Here we propose the use of a spatially-resolved technique, the X-Ray PhotoEmission Electron Microscopy (XPEEM), to gain information on single particles of the composite electrode at different stages of cycling. By combining nanoscale microscopy capabilities with local spectroscopic features within the first 3-4 nm from the surface, we are able to gain unique information on the different particles of the LTO surface. The high lateral resolution achieved with the XPEEM on a LTO electrode is attested in Figure 1.A showing the elemental distribution of the active material (blue), carbon (red) and binder (green) particles. The local XAS spectra acquired at the C K-edges on carbon (Figure 1.B) and LTO (Figure 1.C) areas provide a straightforward signature of their different reactivity upon cycling. Indeed, we demonstrate the presence of a surface layer solely on top of the LTO particles and only on lithiated samples. Thus, we provide an innovative solution to study the complex electrode/electrolyte interaction and propose a mechanism that explains the surface layer formation and the distribution across the LTO electrode, opening the way to achieving an optimized electrode and electrolyte engineering. Figure 1

Investigation into the Mechanism and Causes of Gassing in Li4Ti5O12 Based/ LiMn2O4 Li-Ion Battery Systems

Li4Ti5O12 (LTO) as an anode material for lithium ion batteries has garnered attention as a possible alternative to graphite anodes in the hybrid and electric vehicle auto industry due to its increased safety properties, high rate capabilities, and exceptional cycling stability. The main concern arises in that cells using LTO anodes produce gas especially at elevated temperatures leading to swelling and hampered performance. Many mechanisms have been proposed as possible sources of the gas evolution such as breakdown of electrolyte salts, water, and LTO surface reactions, but no direct cause or solution has been agreed upon. In the open literature, efforts have been made to combat this with either coating on LTO materials2,4 or electrolyte technology without a full understanding of how the material behaves in the cell. In this work, we take the investigation further to pinpoint the causes and mechanisms of the gas evolution in LTO based cells through gas analysis, electrochemical performance, and surface analysis in multilayer pouch cells and full cell coin cells. These results will be discussed in detail with the intention of shedding light on the gas evolution issue with the hope of leading to optimized solutions to resolve the gassing issue. References Y. He, B. Li, M. Liu, C. Zhang, W. Lv, C. Yang, J. Li, H. Du, B. Zhang, Q. Yang, J. Kim, F. Kang, Sci. Rep. 2, 913 (2012).L. Wang, K. Nakura, M. Imazaki, N. Kakizaki, K. Ariyoshi, and T. Ohzuku, J. Electrochem. Soc., 159, A1710 (2012).J. Liu, P. Bian, J. Li, W. Ji, H. Hao, A. Yu, J. Pow. Sour., 286 (2015)W. Li, X. Li, M. Chen, Z. Xie, J. Zhang, S. Dong, M. Qu, Elect. Acta., 139 (2014)J. Gao, B.Gong, Q. Zhang, G. Wang, Y. Dai, W. Fan, Ionics, 21 (2015)J. Liu, P. Bian, J. Li, W. Ji, H. Hao, A. Yu, J. Pow. Sour., 286 (2015)

Enhanced electrochemical performance of core-shell Li4Ti5O12/PTh as advanced anode for rechargeable lithium-ion batteries

A nanocomposite of Li4Ti5O12 particles coated with polythiophene (PTh) was fabricated as advanced anode for rechargeable lithium-ion batteries. The conducting PTh layer was successfully coated on the surface of Li4Ti5O12 through the in-situ oxidative polymerization method. Benefiting from the core-shell structure, specific capacities as high as 171.5, 168.2 and 151.1mAhg−1 at 0.2, 1 and 10 C are obtained in the Li4Ti5O12/PTh composite. The electrochemical results also show that the Li4Ti5O12/PTh exhibits remarkably improved cycling performance as compared with the Li4Ti5O12 anode. Moreover, the charge-transfer resistance of Li4Ti5O12/PTh electrode is much lower than that of the bare Li4Ti5O12, revealing that the PTh coating can significantly increase the electron conductivity between the Li4Ti5O12 particles. The excellent electrochemical performance of the as-fabricated Li4Ti5O12/PTh composite can be ascribed to the PTh layer which can suppress the dissolution of active material into the LiPF6 electrolyte and enhance the electron conductivity of Li4Ti5O12 nanocrystals. Thus, the Li4Ti5O12/PTh composite is an advanced anode for use in high performance lithium-ion batteries application.

Structure and enhanced electrochemical performance of the CaF2-modified Li4Ti5O12 anode material

The surface of pure Li4Ti5O12 was modified with CaF2 via a chemical co-precipitation method followed by calcination at 400°C for 5h in order to study the reaction mechanism of the fluoride modification process. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were used to confirm the structure and morphology of the material. After the modification, the structure of Li4Ti5O12 was unchanged, as F− ions reacted with Ca2+ ions to form CaF2 instead of reacting with Li4Ti5O12. A consecutive CaF2 coating layer was not formed on the surface of Li4Ti5O12; instead, many microscale CaF2 crystals stacked on the Li4Ti5O12 particles. The stacked CaF2 crystals could not only reduce electrode polarization, but also partially suppress reductive decomposition, resulting in the formation of a thinner SEI film, thereby reducing resistance of the SEI film (Rsei) and the charge-transfer resistance (Rct), both of which were beneficial to the electrochemical performance of Li4Ti5O12. The 2wt% CaF2-modified Li4Ti5O12 exhibited a superior rate capacity, with discharge capacities between 0 and 3V of 227.8, 215.8, 207.6, 190.6, 180.2, and 157.6mAhg−1 at rates of 0.2, 0.5, 1, 3, 5, and 10 C (1 C=250mAhg−1), respectively. Moreover, the 2wt% CaF2-modified Li4Ti5O12 showed a discharge capacity retention of 96.7% after 125cycles at a rate of 3 C, higher than that of pure Li4Ti5O12 (94.4%), which was due to the fact that CaF2 modification enhanced the capability of the material to resist HF attack during the discharge/charge processes.

Ionic Conductor of Li2SiO3 as an Effective Dual-Functional Modifier To Optimize the Electrochemical Performance of Li4Ti5O12 for High-Performance Li-Ion Batteries.

Ionic conductor of Li2SiO3 (LSO) was used as an effective modifier to fabricate surface-modified Li4Ti5O12 (LTO) via simply mixing followed by sintering at 750 °C in air. The electrochemical performance of LTO was enhanced by merely adjusting the mass ratio of LTO/LSO, and the LTO/LSO composite with 0.51 wt % LSO exhibited outstanding rate capabilities (achieving reversible capacities of 163.8, 157.6, 153.1, 147.0, and 137.9 mAh g-1 at 100, 200, 400, 800, and 1600 mA g-1, respectively) and remarkable long-term cycling stability (120.2 mAh g-1 after 2700 cycles with a capacity fading rate of only 0.0074% per cycle even at 500 mA g-1). Combining structural characterization with electrochemical analysis, the LSO coating coupled with the slight doping effect adjacent to the LTO surface contributes to the enhancement of both electronic and ionic conductivities of LTO.