3D macroporous electrode and high-performance in lithium-ion batteries using SnO2 coated on Cu foam

A three-dimensional porous architecture makes an attractive electrode structure, as it has an intrinsic structural integrity and an ability to buffer stress in lithium-ion batteries caused by the large volume changes in high-capacity anode materials during cycling. Here we report the first demonstration of a SnO2-coated macroporous Cu foam anode by employing a facile and scalable combination of directional freeze-casting and sol-gel coating processes. The three-dimensional interconnected anode is composed of aligned microscale channels separated by SnO2-coated Cu walls and much finer micrometer pores, adding to surface area and providing space for volume expansion of SnO2 coating layer. With this anode, we achieve a high reversible capacity of 750 mAh g−1 at current rate of 0.5 C after 50 cycles and an excellent rate capability of 590 mAh g−1 at 2 C, which is close to the best performance of Sn-based nanoscale material so far.


Details on stability evaluation in oxidation of the 3D Cu foam
In both electrodes of the dried SnO 2 /Cu foam and the annealed SnO 2 /Cu foam, the copper oxides (CuO and Cu 2 O) being possibly formed through wet synthesis route such as sol-gel, were not observed from XRD analysis as shown in Supplementary Fig. S5. However, due to the detection limitation of XRD analysis, trace of the CuO and Cu 2 O could be developed in the SnO 2 /Cu foam electrode. Therefore, XPS analysis was conducted to examine the surface of Cu foam in the annealed SnO 2 /Cu foam as the final product.

I. CuO
The XPS profile of annealed SnO 2 /Cu foam is presented in Supplementary Fig. S5c. The two peaks located at around 934.9 eV and 954.8 eV are assigned to the binding energy of Cu 2p 3/2 and Cu 2p 1/2 , respectively, which indicates the presence of Cu 2+ (CuO) in the SnO 2 /Cu foam. In addition, the shake-up satellite peak at a binding energy approximately 9 eV higher than that of Cu2p 3/2 further confirms the existence of the Cu 2+ on the surface of SnO 2 /Cu foam R1,2 . XPS depth profiles with Ar ion beam etching were used to further probe the chemical oxidation state of SnO 2 /Cu foam. The surface oxide layer of CuO is not detected after Ar ion etching after from 10 s to 120 s, confirming that the Cu foam maintains the metallic character under the thin surface oxide layer after the SnO 2 sol-gel coating. Figure S5 (c) XPS profiles of SnO 2 /Cu foam with various Ar ion etching time.

Supplementary
By using the XPS analysis operating condition (240 keV, 1 µA, and 2ⅹ2 mm 2 ) and the below equation, the etching rate could be estimated R3,4 . The etching rate is calculated at 1.104 Å /s And, the thickness of CuO during 10 s is about 1.1 nm.
The volume of CuO on the Cu foam can be estimated by using the surface area of Cu foam ** and the thickness of CuO layer. ** The surface area could be calculated the specific surface area and the mass of Cu foam.
ⅹ ⅹ ⅹ ⅹ From the 0.00407ⅹ10 -9 m 3 of V CuO and the 6.315 g/cm 3 of ρ CuO , the mass of CuO on Cu foam is calculated at 0.0000257 g in the SnO 2 /Cu foam electrode.

II. Cu 2 O
In the case of Cu 2 O, distinguishing it from metallic Cu in same XPS pattern is not easy due to the negligible difference of 0.2 eV between Cu2p 3/2 binding energies of Cu 2 O and Cu R6 .
Although the quantitative analysis of Cu 2 O through XPS analysis is difficult, by using XRD detection limitation (2~3 wt% with laboratory X-ray source and 0.1 wt% with synchrotron radiation source) R7 , approximately 3 wt% of the sample not observed in XRD pattern may be exist to maximum value in the SnO 2 /Cu foam. When the mass of sample for XRD analysis is 0.002336 g, about 0.0000701 g of Cu 2 O is in the SnO 2 /Cu foam electrode.
The mass contribution of CuO and Cu 2 O in total active material is approximately 2 wt% and 5 wt%, respectively.
Supplementary Figure S6