Size‐Controlled Boron‐Based Bifunctional Photocathodes for High‐Efficiency Photo‐Assisted Li–O2 Batteries

Abstract Photo‐assisted Li–O2 batteries are introduced as a promising strategy for reducing severe overpotential by directly employing photocathodes. Herein, a series of size‐controlled single‐element boron photocatalysts are prepared by the meticulous liquid phase thinning methods by combining probe and water bath sonication, and their bifunctional photocathodes in the photo‐assisted Li–O2 batteries are systematically investigated. The boron‐based Li–O2 batteries have shown incremental round‐trip efficiencies as the sized reduction of boron under illumination. It is noteworthy that the completely amorphous boron nanosheets (B4) photocathode not only delivers an optimizing round‐trip efficiency of 190% on the basis of the ultra‐high discharge voltage (3.55 V) and ultra‐low charge voltage (1.87 V) but also gives a high rate performance and ultralong durability with a round‐trip efficiency of 133% after 100 cycles (200 h) compared with the other‐sized boron photocathodes. This remarkable photoelectric performance of the B4 sample can be attracted to the synergistic effect on the suitable semiconductor property, high conductivity, and strengthened catalytic ability of boron nanosheets coated with ultrathin amorphous boron‐oxides overlayer. This research can open a new avenue to facilitate the rapid development of high‐efficiency photo‐assisted Li–O2 batteries.


Preparation of various-sized boron samples
The liquid phase stripping method has been improved by combining the probe and water bath sonication with a differential centralization technique to prepare boron samples with different sizes from the micron to nanometer level. Detailed operation as follows. Firstly, 120 mg bulk boron powder was grind for 30 min (B1), and then directly added into 60 mL acetonitrile solvent to form the suspension with an initial concentration of 2 mg/mL. Next, the suspension was treated by the probe sonication at a power of 400 W for 6 h with the sonication of 2 s and pause of 3 s. To obtain differentsized boron samples, the as prepared B/acetonitrile solution was carried out by different centrifugation rates and water bath sonication processes. Specifically, the B/acetonitrile suspension was firstly centrifuged at a low speed of 1500 rpm for 10 min to obtain the sediment (B2) and supernatant, and subsequently the supernatant was centrifuged at a high speed of 6000 rpm for 20 min to obtain another sediment (B3) and supernatant (B4). It is worth noting that the preparation of boron quantum dots is slightly different from the above methods. First, the boron powder wasn't required for grinding treatment, the suspension of B/acetonitrile after probing ultrasound for 6 h was centrifuged at 6000 rpm for 20 min and followed by treating by bath sonication for 1 h, boron quantum dots (B5) was finally prepared. Noted that both the probe and water bath sonication processes were conducted under a constant temperature of 0~5 °C . All kind of products were followed by drying in vacuum at 50 °C for overnight to obtain the final sample.

Materials characterization
The crystallographic structures were characterized by using powder X-ray

Assembling and testing of the Li-O2 batteries
As-prepared boron products, Super-P, and polyvinylidene fluoride (PVDF) were mixed together at 8:1:1 weight ratio, grinded for 30 min to blend uniformly, and then coated on a clean nickel foams with a diameter of 13 mm, dried in a vacuum oven at 80 °C for overnight. The mass loading of active materials per electrode was 10 mg/cm -2 . CR2032 coin cells with holes on the cathode side were used for testing Li-O2 batteries.
The batteries were assembled in a glove box and filled at Ar atmosphere with the moisture and oxygen content of below 0.1 ppm. Li foils were used as the counter, boron cathode was as an oxygen electrode and photoelectrode, and glass fiber (Waterman, GF/A) was applied as the separators. 1.0 M LiTFSI in tetraethylene glycol dimethyl ether (Tetraglyme) was employed as the electrolyte. The assembled cells were stored in a volume capacity of 200 mL sealed glass test device, which was filled with oxygen.
Significantly, the oxygen filled glass test devices were purged with oxygen for a few minutes to remove residual gases before being used. Finally, all of the cells were tested on a CHI 760E electrochemical workstation (Shanghai, China). The GEL S500/350 Xe-lamp (CEAULIGHT, BEIJING) was utilized as the solar source for illumination, and the power was fixed at 500 W.

Theoretical calculation methods
The Vienna Ab Initio Package (VASP) had been employed to perform all spin- And, only the Γ point was used to sample the first Brillouin zone for B28 clusters and B20O8 clusters. A geometry optimization was considered convergent when the energy change was smaller than 0.02 eV Å -1 . The weak interaction was described by DFT+D3 method using empirical correction in Grimme's scheme. The Gibbs free energy for each elementary step was calculated as: G = Eelec + EZPE -TS in which Eelec was the electronic energy at 0 K calculated by DFT, EZPE was the zeropoint energy term, T was the absolute temperature (here 298.15 K), and S was the entropy.                EIS was performed to investigate the kinetics of Li + diffusion process in the electrode.
The Li + diffusion coefficients were obtained based on the Nyquist plots. The Li + diffusion coefficient can be calculated using the following equation: where R is the gas constant with value 8.314 J K −1 mol −1 , T is the room Kelvin temperature of 298 K, A is the electrode area, which is 0.149 cm 2 , n is the number of the electrons per 2 molecule attending the charge and discharge process, F is the faraday constant (96500 C mol -1 ), C is the concentration of lithium ion of Li2O2 in boron electrode, which is 0.0263 mol cm -3 , σ is the slope of the line Z'-ω -1/2 , B4 and B5 are