LiCoPO4 cathode from a CoHPO4·xH2O nanoplate precursor for high voltage Li-ion batteries

A highly crystalline LiCoPO4/C cathode material has been synthesized without noticeable impurities via a single step solid-state reaction using CoHPO4·xH2O nanoplate as a precursor obtained by a simple precipitation route. The LiCoPO4/C cathode delivered a specific capacity of 125 mAhg−1 at a charge/discharge rate of C/10. The nanoplate precursor and final LiCoPO4/C cathode have been characterized using X-ray diffraction, thermogravimetric analysis − differential scanning calorimetry (TGA-DSC), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) and the electrochemical cycling stability has been investigated using different electrolytes, additives and separators.


Introduction
Li-ion batteries − widely applied as an energy storage system of choice for [ 6 _ T D $ D I F F ] electric vehicles, as well as for large scale stationary applications − have the highest energy density amongst the many types of proposed and commercialized rechargeable batteries [1,2,3,4,5,6]. Such a high energy density is attained, in part, by both the high specific capacity and voltage of the cathode electrode. Other than the conventionally used oxide-based cathodes, phosphate polyanion-type cathodes have been widely investigated. A notable example is the commercialization of LiFePO 4 which is one of the most stable cathode materials available due to its unique olivine structure [7,8,9,10,11,12]. Among the olivine phosphate-based cathodes (LiMPO 4 : M: Fe, Mn, Co and Ni), LiCoPO 4 possesses a high redox potential of 4.8 V vs. Li/Li + , a flat voltage profile, and a high theoretical capacity of 167 mAhg −1 [13]. However, efforts to utilize LiCoPO 4 thus far have shown limited capacity and fast fading [ 7 _ T D $ D I F F ] of the capacity upon repetitive cycles [13,14,15]. Like other phosphates, to access the full specific capacity from a LiCoPO 4 cathode, [ 8 _ T D $ D I F F ] a nanostructured synthesis of the active material is desired [13,16,17,18].
Various methods have been developed for LiCoPO 4 cathode synthesis including precipitation, hydrothermal, microwave, solid-state, mechanochemical, supercritical fluid and spray drying [14,16,17,18,19,20,21,22,23]. However, many of the synthesis routes reported are not suitable for scale-up and require complicated heat-treatment steps to ensure the formation of pure stoichiometric LiCoPO 4 since many of the available Co precursor can be easily reduced to form impurities such as Co metal, Co 3 O 4 and Li 3 PO 4 phases.
Previously, NH 4 CoPO 4 nanoplates were used as a [ 9 _ T D $ D I F F ] starting material for LiCoPO 4 , but multiple heat-treatments in both air and inert atmosphere were required to ensure formation of stoichiometric LiCoPO 4 since H 2 produced during the decomposition of NH 4 CoPO 4 generates Co metal [14]. Other metal organic compounds are also prone to produce Co metal during heat-treatment by carbothermal reduction.
To form [ 1 0 _ T D $ D I F F ] a stoichiometric LiCoPO 4 cathode without impurities, precursor compounds with strong Co-P bonding [ 1 1 _ T D $ D I F F ] are desired. In the present work, a nanostructured CoHPO 4 ·xH 2 O precursor was used to simplify the synthesis process and to minimize impurities. Previously, in related work, a CoHPO 4 ·3H 2 O nanosheet electrode was hydrothermally synthesized for supercapacitors applications [24]. Finally, the effect of the electrolyte and separator on the cycling stability of the LiCoPO 4 /C cathode obtained was investigated.

Experimental
The LiCoPO 4 cathode was synthesized by a solid-state reaction using LiOH, were mixed with 4.04 wt% (i.e., 5 wt% relative to LiCoPO 4 ) of Ketjen black using a planetary mill (Retch 200CM) for 4 h followed by heat-treatment in a tube furnace at 700°C for 10 h under an UHP-Ar atmosphere with a heating rate of 5°C min −1 .
A simultaneous differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) system (Netzsch STA 449C Jupiter) equipped with a SiC high temperature furnace (25-1550°C) and a type-S sample holder was used to study the dehydration and phase transformation of the CoHPO 4 ·xH 2 O nanoplate precursor. The powder sample was heated in an air environment up to 700°C at a ramp rate of 5°C min −1 . The crystal structure of the as-prepared LiCoPO 4 /C composite was determined by X-ray diffraction (XRD) using a Rigaku Mini-Flex II with a CuKα sealed tube (λ = 1.54178 Å). All of the samples were scanned in a 2θ range between 5 to 80°, with a step size of 0.01°and an exposure time of 30 s. A JEOL 7001F scanning electron microscope (SEM) system was used to investigate the particle morphology. A high-resolution transmission electron microscopic (HRTEM) analysis was conducted using a FEI Tecnai G2 microscope with an acceleration voltage of 200 kV.
Electrodes were prepared by casting a slurry of the LiCoPO 4 /C composite, acetylene black (MTI), and polyvinylidenedifluoride (PVDF, MTI) in N-methylpyrrolidone (NMP: Aldrich) solvent onto an Al foil current collector.
The total weight percentage of carbon and PVDF in the electrode was 10 wt%

Results and discussion
To synthesize the LiCoPO 4 nanoparticles, the CoHPO 4 ·xH 2 O nanoplate precursor was obtained by a precipitation reaction between Co 2+ and P 2 O 7 4− (from Na 2 H 2 P 2 O 7 ) in acidic media of pH 5 ∼ 6 at 80°C for 8 h resulting in a violet CoHPO 4 ·xH 2 O powder. Fig. 1(a, b) shows the powder XRD patterns of  is the stable phase, but [ 1 8 _ T D $ D I F F ] transforms to α-Co 2 P 2 O 7 as the temperature is decreased to room temperature [25,26]. Overall, the dehydration and phase    Fig. 2(a)  with only H 2 O as a by-product resulting in uniform nanoparticles without much grain growth.  reacts with PF 6 − , POF 3 and PO 2 F 2 − to produce more HF [15,27]. Therefore, to prevent CoPO 4 dissolution during cycling, HF should ideally be eliminated − which is a challenging task. Various strategies have been tested to stabilize the cycling performance including the use of an HF scavenging separator, protective coating, and doping to induce SEI (solid electrolyte interphase) layer formation using electrolyte additives [15,28]. Using the latter approach, Fe-substituted LiCoPO 4 exhibited an improved cycling stability due to the stabilization of the structure in the delithiated state [29,30]. However, a lower  various electrolyte additives have also improved the cycling performance.

[ ( F i g . _ 3 ) T D $ F I G ]
An improved capacity retention was observed when LiCoPO 4 was cycled with an electrolyte containing either tris(hexafluoroisopropyl) phosphate (HFiP) or trimethylboroxine (TMB) [31]. Additionally, the use of alternative separators such as glassy paper or quartz has increased the cycling stability relative to the conventional polyethylene(PE)/polypropylene (PP) separators due to the presence of silica, which is known to be a HF scavenger.
The cyclic performance of the LiCoPO 4 /C cathode is shown in Fig. 4(b). The large irreversible losses in the capacities observed at the beginning of the cycles are believed to be due to the SEI layer formation by the decomposition of the electrolyte and the additives. When a conventional 1 M LiPF 6 in EC:DMC (1:1 v/v) electrolyte was used, ∼50% and ∼80% degradation in the specific capacity after 10 and 50 cycles has been observed, respectively,[ 4 0 _ T D $ D I F F ] which is similar to previous reports [14,28]. However, when a glassy separator and a 1 M LiPF 6 in FEC:DMC (1:4 v/v) electrolyte with 1.5 wt% TMB additive was used, over 90% and 60% of the initial capacity has been retained after 10 and 50 cycles, respectively. The FEC-based electrolyte with the TMB additive demonstrates a dramatic improvement in the cycling characteristics of the LiCoPO 4 /Li cells as compared to the EC-based electrolyte. Achieving an electrolyte with high voltage stability and HF minimization is a challenging task. Further investigations on the cycling stability of the cathode are currently ongoing and detailed information regarding the influence of the electrolyte formulations on the LiCoPO 4 cycling stability will be reported in the future.

Conclusions
A highly stoichiometric LiCoPO 4 /C cathode material has been synthesized using a CoHPO 4 ·xH 2 O precursor obtained by a simple precipitation route at room temperature which is suitable for a large scale synthesis. The CoHPO 4 ·xH 2 O obtained has a nanoplate shape morphology with a x = 1 hydration level. A pure, stoichiometric LiCoPO 4 /C cathode was obtained by a single step heat-treatment at 700°C which delivers a specific capacity of 125 mAhg −1 at a C/10 rate containing 10 wt% conductive carbon additive indicating that the CoHPO 4 ·xH 2 O precursor is an ideal starting material for LiCoPO 4 cathode synthesis. With a variation in the composition of a carbonate-based electrolyte and use of an additive, a significant improvement in the cycling stability was observed. It is likely that, with a more systematic understanding of the degradation mechanism(s) and further electrolyte optimization, the cycling performance of the high voltage LiCoPO 4 cathode can be significantly improved.