Researchers discover how nickel may inhibit charge/discharge rate in Li-ion batteries

Left. Image of LMNO surface layer.Upper right. Simulated zone projection image based on LMNO crystal model with 20% Ni/Li disorder corresponding to blue rectangle. Lower right. Simulated zone projection image based on LMNO crystal model with 10% Ni/Li disorder corresponding to white rectangle. TM = transition metal. Credit: ACS, Gu et al. Click to enlarge.

A research team from the US Department of Energy’s (DOE) Pacific Northwest National Laboratory (PNNL) and Argonne National Laboratory (ANL), with colleagues from FEI, Inc. and Binghamton University, has discovered a thermodynamically driven, yet kinetically controlled, surface modification in lithium-nickel-manganese-oxide (LMNO) cathode material which may inhibit the battery charge/discharge rate.

Examining battery materials on the nano-scale, they found how nickel forms a physical barrier that impedes the shuttling of lithium ions in the electrode, reducing how fast the materials charge and discharge. Published last week in the ACS journal Nano Letters, the research also suggests a way to improve the materials.

Lithium transition-metal oxides have been widely used as the cathode for Li ion batteries. They can be tailored to gain either high voltage or high capacity by adjusting the relative ratio of different transition-metal ions and preparation conditions. For example, a layered composite based on lithium nickel manganese oxide Li_1.2Ni_0.2Mn_0.6O₂ (LNMO) has demonstrated a rechargeable capacity of >250 mAh/g, which is much larger than that of the conventional LiCoO₂ cathode (<140 a="a" act="act" as="as" by="by" can="can" category="category" channels="channels" charge="charge" composite="composite" discharge="discharge" during="during" em="em" featured="featured" for="for" g="g" in="in" ions="ions" is="is" layered="layered" li="li" low-barrier="low-barrier" mah="mah" material="material" move="move" of="of" path="path" processes.="processes." structure="structure" the="the" this="this" to="to" which="which" within="within">

Here we report our surprising discovery of a selective surface lattice plane segregation of nickel (Ni) ions for the case of LNMO as a representative case for the transition-metal oxide-based cathode and the possible implications of such a surface segregation on the Li ion transport behavior in this category of cathode material. What we have observed is a phenomenon that is far beyond general expectation and will broadly impact the research effort for enhancing the rate performance of Li ion batteries and stability of cathode in the electrolyte.

—Gu et al.

The researchers, led by PNNL’s Chongmin Wang, created high-resolution 3D images of electrode materials made from lithium-nickel-manganese oxide layered nanoparticles, mapping the individual elements. These maps showed that nickel formed clumps at certain spots in the nanoparticles. A higher magnification view showed the nickel blocking the channels through which lithium ions normally travel when batteries are charged and discharged.

We were surprised to see the nickel selectively segregate like it did. When the moving lithium ions hit the segregated nickel-rich layer, they essentially encounter a barrier that appears to slow them down. The block forms in the manufacturing process, and we’d like to find a way to prevent it.

—Chongmin Wang

In lithium-manganese oxide electrodes, the manganese and oxygen atoms form rows like a field of cornstalks. In the channels between the stalks, lithium ions zip towards the electrodes on either end, the direction depending on whether the battery is being used or being charged.

Researchers have known for a long time that adding nickel improves energy capacity and voltage, but haven’t understood why the capacity falls after repeated usage.

The researchers used electron microscopy at the Environmental Molecular Sciences Laboratory (EMSL) at PNNL and the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory to view how the different atoms are arranged in the electrode materials produced by Argonne National Laboratory researchers. The electrodes were based on nanoparticles made with lithium, nickel, and manganese oxides.

First, the team took high-resolution images that clearly showed rows of atoms separated by channels filled with lithium ions. On the surface, they saw the accumulation of nickel at the ends of the rows, essentially blocking lithium from moving in and out.

To find out how the surface layer is distributed on and within the whole nanoparticle, the team used a technique called three-dimensional composition mapping. Using a nanoparticle about 200 nanometers in size, they took 50 images of the individual elements as they tilted the nanoparticle at various angles. The team reconstructed a three-dimensional map from the individual elemental maps, revealing spots of nickel on a background of lithium-manganese oxide.

The three-dimensional distribution of manganese, oxygen and lithium atoms along the surface and within the particle was relatively even. The nickel, however, parked itself in small areas on the surface. Internally, the nickel clumped on the edges of smaller regions called grains.

This data visualization shows how manganese (blue) and nickel (green) are distributed in a nanoparticle about 200 nanometers tall. Nickel’s uneven distribution affects the energy capacity of the battery electrode made from these nanoparticles in lithium-ion batteries.

To explore why nickel aggregates on certain surfaces, the team calculated how easily nickel and lithium traveled through the channels. Nickel moved more easily up and down the channels than lithium. While nickel normally resides within the manganese oxide cornrows, sometimes it slips out into the channels. And when it does, this analysis showed that it flows much easier through the channels to the end of the field, where it accumulates and forms a block.

The researchers used a variety of methods to make the nanoparticles. Wang said that the longer the nanoparticles stayed at high temperature during fabrication, the more nickel segregated and the poorer the particles performed in charging and discharging tests. They plan on doing more closely controlled experiments to determine if a particular manufacturing method produces a better electrode.

This work was supported by PNNL’s Chemical Imaging Initiative.

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