Youhao Liao

Layered Lithium-Rich Oxide Nanoparticles Doped with Spinel Phase: Acidic Sucrose-Assistant Synthesis and Excellent Performance as Cathode of Lithium Ion Battery.

Nanolayered lithium-rich oxide doped with spinel phase is synthesized by acidic sucrose-assistant sol-gel combustion and evaluated as the cathode of a high-energy-density lithium ion battery. Physical characterizations indicate that the as-synthesized oxide (LR-SN) is composed of uniform and separated nanoparticles of about 200 nm, which are doped with about 7% spinel phase, compared to the large aggregated ones of the product (LR) synthesized under the same condition but without any assistance. Charge/discharge demonstrates that LR-SN exhibits excellent rate capability and cyclic stability: delivering an average discharge capacity of 246 mAh g(-1) at 0.2 C (1C = 250 mA g(-1)) and earning a capacity retention of 92% after 100 cycles at 4 C in the lithium anode-based half cell, compared to the 227 mA g(-1) and the 63% of LR, respectively. Even in the graphite anode-based full cell, LR-SN still delivers a capacity of as high as 253 mAh g(-1) at 0.1 C, corresponding to a specific energy density of 801 Wh kg(-1), which are the best among those that have been reported in the literature. The separated nanoparticles of the LR-SN provide large sites for charge transfer, while the spinel phase doped in the nanoparticles facilitates lithium ion diffusion and maintains the stability of the layered structure during cycling.

Sulfur loaded in curved graphene and coated with conductive polyaniline: preparation and performance as a cathode for lithium–sulfur batteries

We report a composite (CG-S@PANI), sulfur (S) loaded in curved graphene (CG) and coated with conductive polyaniline (PANI), as a cathode for lithium–sulfur batteries. CG is prepared by splitting multi-wall carbon nanotubes and loaded with S via chemical deposition and then coated with polyaniline via in situ polymerization under the control of ascorbic acid. The physical and electrochemical performances of the resulting CG-S@PANI are investigated by nitrogen adsorption–desorption isotherms, X-ray powder diffraction, thermogravimetric analysis, transmission electron microscopy, electrochemical impedance spectroscopy, charge–discharge tests, and electronic conductivity measurements. CG-S@PANI as a cathode for lithium–sulfur batteries delivers an initial discharge capacity of 851 mA h g−1 (616 mA h g−1 on the basis of the cathode mass) at 0.2 C with a capacity retention of over 90% after 100 cycles. This nature is attributed to the co-contribution of CG and conductive PANI to the concurrent improvement in electronic conductivity and chemical stability of the sulfur cathode.

Structural Exfoliation of Layered Cathode under High Voltage and Its Suppression by Interface Film Derived from Electrolyte Additive

Layered cathodes for lithium-ion battery, including LiCo1-x-yNixMnyO2 and xLi2MnO3·(1-x)LiMO2 (M = Mn, Ni, and Co), are attractive for large-scale applications such as electric vehicles, because they can deliver additional specific capacity when the end of charge voltage is improved to over 4.2 V. However, operation under a high voltage might cause capacity decaying of layered cathodes during cycling. The failure mechanisms that have been given, up to date, include the electrolyte oxidation decomposition, the Ni, Co, or Mn ion dissolution, and the phase transformation. In this work, we report a new mechanism involving the exfoliation of layered cathodes when the cathodes are performed with deep cycling under 4.5 V in the electrolyte consisting of carbonate solvents and LiPF6 salt. Additionally, an electrolyte additive that can form a cathode interface film is applied to suppress this exfoliation. A representative layered cathode, LiCoO2, and an interface film-forming additive, dimethyl phenylphosphonite (DMPP), are selected to demonstrate the exfoliation and the protection of layered structure. When evaluated in half-cells, LiCoO2 exhibits a capacity retention of 24% after 500 cycles in base electrolyte, but this value is improved to 73% in the DMPP-containing electrolyte. LiCoO2/graphite full cell using DMPP behaves better than the Li/LiCoO2 half-cell, delivering an initial energy density of 700 Wh kg -1 with an energy density retention of 82% after 100 cycles at 0.2 C between 3 and 4.5 V, as compared to 45% for the cell without using DMPP.

The improved effect of co-doping with nano-SiO2 and nano-Al2O3 on the performance of poly(methyl methacrylate-acrylonitrile-ethyl acrylate) based gel polymer electrolyte for lithium ion batteries

In this article, we report a novel gel polymer electrolyte (GPE) for lithium ion batteries, which is prepared using poly(methyl methacrylate-acrylonitrile-ethyl acrylate) (P(MMA-AN-EA)) as a polymer matrix and doping with nano-SiO2 and nano-Al2O3 simultaneously. The influences of the ratio of the two nanoparticles on the pore structure, electrolyte uptake and thermal stability of the resulting membrane, and the ionic conductivity and electrochemical stability of the corresponding GPE are investigated by scanning electron microscopy, mechanical strength, thermogravimetry, electrochemical impedance spectroscopy, linear sweep voltammetry and cyclic voltammetry. The performance of the developed GPE is evaluated in the Li/LiNi0.5Mn1.5O4 half cell by a charge–discharge test for its application in lithium ion batteries. It is found that there exists a synergistic effect between nano-SiO2 and nano-Al2O3. The performances of the resulting membrane and the corresponding GPE are effectively improved by using nano-SiO2 and nano-Al2O3 simultaneously rather than individually. Co-doping 5 wt% nano-SiO2 and 5 wt% nano-Al2O3 provides the membrane with a higher thermal decomposition temperature of 325 °C, and a better electrolyte uptake of 198.1%, the corresponding GPE with an increased ionic conductivity of 2.2 × 10−3 S cm−1 at room temperature and an enhanced oxidative stability up to 5.5 V (vs. Li/Li+), and the LiNi0.5Mn1.5O4 cathode with an improved rate capability of 104.2 mA h g−1 at 2C and an improved capacity retention of 94.8% after 100 cycles. These improved performances result from combining the advantages of both nano-SiO2 and nano-Al2O3, in which the former contributes to the improved ionic conductivity caused by a stronger Lewis-acid property, while the latter to the better thermal and structural stabilities by its stiffness characteristic.

Constructing Unique Cathode Interface by Manipulating Functional Groups of Electrolyte Additive for Graphite/LiNi0.6Co0.2Mn0.2O2 Cells at High Voltage

A novel electrolyte additive, 1-(2-cyanoethyl) pyrrole (CEP), has been investigated to improve the electrochemical performance of graphite/LiNi0.6Co0.2Mn0.2O2 cells cycling up to 4.5 V vs Li/Li+. The 4.5 V cycling results present that after 50 cycles, up to 4.5 V capacity retention of the graphite/LiNi0.6Co0.2Mn0.2O2 cell is improved significantly from 27.4 to 81.5% when adding 1% CEP to baseline electrolyte (1 M LiPF6 in EC/EMC 1:2, by weight). Ex situ characterization results support the mechanism of CEP for enhancing the electrochemical performance. On one hand, the significant enhancement is ascribed to a formed superior cathode interfacial film by preferential oxidation of CEP on the cathode electrode surface suppressing electrolyte decomposition at high voltage. On the other hand, the duo Lewis base functional groups can effectively capture dissociation product PF5 from LiPF6 with the presence of an unavoidable trace amount of water or aprotic impurities in the electrolyte. Thus this mitigates the hydrofluoric acid (HF) generation that leads to the reduction of transition-metal dissolution in the electrolyte upon cycling at high voltage. The theoretical modeling suggests that CEP has a mechanism of stabilizing electrolyte via combination of -C≡N: functional group and H2O. The work presented here also shows nuclear magnetic resonance spectra analysis to prove the capability of CEP reducing HF generation and X-ray photoelectron spectroscopy analysis to observe cathode surface composition.

Diethyl(thiophen-2-ylmethyl)phosphonate: a novel multifunctional electrolyte additive for high voltage batteries

Carbonate-based electrolytes used in Li-ion batteries encounter various challenges in extreme electrochemical environments, and hence their application requires various additives, especially when used with high voltage cathode materials. These additives are designed to form protective interphases that prevent parasitic carbonate oxidation, while in certain cases they stabilize electrolytes from reduction at anode surfaces or even serve as flame-retardants that postpone the thermal runaway during overcharge. However, most of these additives casts negative effects, lowering ionic conductivity of electrolyte or impairing the compatibility between cathode and electrolyte. An ideal solution of minimizing the presence of these inert molecules is to identify an additive that structurally integrates these multiple functions into a single compound. In this work, we report a novel additive, diethyl(thiophen-2-ylmethyl)phosphonate (DTYP). Its 0.5% presence in a base electrolyte dramatically improves the capacity retention of a high voltage Li-ion cell using LiNi0.5Mn1.5O4 from 18% to 85% after 280 cycles at 1C at 60 °C, increases the endothermic reaction onset temperature from 193 °C to 223 °C, and reduces the self-extinguishing time of the electrolyte from 88 s to 77 s. Thus, such a multifunctional additive presents a cost-efficient solution to the issues often faced in high voltage lithium-ion batteries.

Mechanism of cycling degradation and strategy to stabilize a nickel-rich cathode

A nickel-rich LiNi0.8Co0.15Al0.05O2 (NCA) cathode possesses high specific capacity and high discharge voltage, as the most promising cathode for high energy density lithium ion batteries, but suffers from serious cycling degradation. The present study revealed that the NCA cathode is stable with excellent cycling stability at voltages below 4.2 V, but suffers from serious degradation at voltages above 4.35 V. The characterization from SEM, TEM, XPS, FTIR, NMR, XRD and ICP as well as electrochemical measurements supported by theoretical calculations revealed that the trace of HF initially present in battery grade electrolytes likely induces the cycling stability degradation of the nickel-rich NCA cathode via accelerating the electrolyte decomposition. Our further research demonstrated that such cycling stability degradation can be eliminated through applying diethyl phenylphosphonite (DEPP) as an electrolyte additive, as DEPP is capable of shielding HF besides its ability to construct a protective cathode interphase, resulting in an excellent cycling stability of the nickel-rich NCA cathode.