Operando View on the Charge Transport through LiFePO4 Electrodes and the Phase Transformation in Individual LiFePO4 Grains Using Neutron Depth Profiling and Microbeam Synchrotron Diffraction
Authors: Zhang Xiaoyu , Verhallen Tomas , Labohm Freek , van Hulzen Martijn , Singh Deepak , Wright Jonathan P. , van Dijk Niels H. , Wagemaker Marnix 
Published: February 2020

Here we present our recent findings using two novel in-situ techniques1,2. (1) The distribution of Li-ions in LiFePO4 electrodes under operando conditions reveals what charge transport mechanism is rate limiting under what conditions including the impact of particle size and carbon additives. (2) In operando micro beam diffraction is performed from slow to high cycling rates disclosing phase transformation mechanism in individual LiFePO4 for the first time.

Neutron Depth Profiling (NDP) offers the possibility to see directly see lithium ions, or atoms, via the capture reaction of neutrons with the 6Li isotope to form an α-particle (4He) and a trition (3H) carrying the  energy that is generated by the reaction according to conservation of energy and momentum. The 4He and 3H particle travel through the surrounding material during which they lose energy. By measuring the particle energy when it reaches the detector, the depth at which the 6Li atom was located can be determined. This allows to reconstruct a Li-atom density profile as a function of depth without significantly influencing the Li concentration due to the low neutron flux making this a non-destructive technique. Advancing the application of NDP from micro batteries3to conventional batteries we show the impact of particle size and charge rate on how the activity in the electrodes is distributed. This gives insight in what transport process dominates the internal resistance under what conditions.

Zooming in on the phase transition reaction within single LiFePO4 grains has been achieved by applying a bright and sub-micron sized synchrotron beam. Under these conditions diffraction rings observed with powder diffraction fall apart in individual spots each representing individual LiFePO4 grains in the positive electrode. In this way the phase transition behavior through time of hundreds of individual 140 nm LiFePO4 grains is monitored at the same time while varying the electrochemical conditions ranging from slow (C/5) up to fast rates (2C). Following the phase transformation kinetics of individual grains reveals much slower transformation rates than expected based on the generally accepted mosaic or “Domino Cascade” transformation. In addition we show that the transformation rate of individual grains actually depends on the cycling rate. The appearance of many streaked reflections observed at low rates discloses that the majority of the grains is actively transforming via nanometer thin plate shaped domains that nucleate in specific crystallographic orientations. Also this observations defies the  long believed mosaic or “domino cascade” transformation model. As the (dis)charge rate increases the number of these plate shaped domains decreases and their width increases, driving the local compositions of the coexisting phases towards each other. The observation of a diffuse interface in a single grain at high (dis)charge rates reveals the growth mechanism at high rates, consistent with recent operando powder diffraction probing the average crystalline state over all grains4,5.

Thereby, a consistent mechanistic picture is revealed for the phase transformation in individual LiFePO4 grains under realistic operando conditions for the first time. Counter intuition, the existence of well-defined interfaces at low discharge rates, may cause a shorter cycle life as compared to the more diffuse interfaces at higher rates. The electrochemically driven higher transformation rates indicate phase transformation kinetics play a more important role at low rates as compared to higher rates where the diffuse interfaces appear to be responsible for the faster transformation. Consistent with the electrolyte transport limited reactions, a smaller faction of the electrode material appears actively transforming, in accordance to the NDP results, indicating this has to be the focus for high rate electrode improvement.

(1) Zhang, X.; Verhallen, T. W.; Labohm, F.; Wagemaker, M. Advanced Energy Materials
2015, 15, 1500498.

(2) Zhang, X.; van Hulzen, M.; Singh, D. P.; Brownrigg, A.; Wright, J. P.; van Dijk, N. H.; Wagemaker, M. Nat Commun
2015, 6.

(3) Oudenhoven, J. F. M.; Labohm, F.; Mulder, M.; Niessen, R. A. H.; Mulder, F. M.; Notten, P. H. L. Advanced Materials
2012, 23, 4103.

(4) Zhang, X.; van Hulzen, M.; Singh, D. P.; Brownrigg, A.; Wright, J. P.; van Dijk, N. H.; Wagemaker, M. Nano Letters
2014, 14, 2279.

(5) Liu, H.; Strobridge, F. C.; Borkiewicz, O. J.; Wiaderek, K. M.; Chapman, K. W.; Chupas, P. J.; Grey, C. P. Science
2014, 344.

Figure 1