Anode - Battery Design

The Anode is the negative or reducing electrode that releases electrons to the external circuit and oxidizes during and electrochemical reaction. In a lithium ion cell the anode is commonly graphite or graphite and silicon.

The cell stack comprises: anode electrode, anode active material, separator, cathode active material, cathode electrode and of course the electrolyte. The cell stack is the basic unit of the working electrochemistry. Inside a cell is a wound or stacked electrochemical system.

Alloying-Type Anode An anode material, such as silicon, tin, antimony, bismuth or germanium, that stores lithium by alloying reaction instead of intercalation, giving high capacity at the cost of large volume expansion on cycling.

Conversion-Type Anode An anode material, typically a metal oxide or sulfide, that reacts with lithium via a conversion reaction instead of intercalation or alloying.

Ande and cathode deifinition in chemistry

Anode-Cathode

Anode and Cathode are not fixed and change positions depending on whether the cell is being charged or discharged. It is therefore incorrect to state that the electrons move from Cathode to Anode during the recharging process. The – and + electrodes (terminals) however stay put. For example, in a typical Lithium ion cobalt oxide battery, graphite is the – electrode and LCO is the + electrode at all times.

Anode Materials

Antimony Anode
- Alloying-type anode material relevant to sodium-ion and potassium-ion batteries, valued for its relatively high theoretical capacity.
Bismuth Anode
- Alloying-type anode material of interest for sodium-ion and multivalent battery chemistries, offering good volumetric capacity.
Germanium Anode
- High-capacity alloying-type anode material with fast lithium diffusion, limited in practice by cost.
Graphite
- Capacity 372mAh/g (theoretical)
LTO
- Lithium Titanate or Lithium Titanium Oxide
- Lower energy density, typically ~80Wh/kg at cell level
- Wider operating temperatures
- Low operating voltage 1.9V to 2.9V
- High discharge rates
MXene Anode
- Material based on two-dimensional transition-metal carbide or nitride sheets, an active area of battery materials research.
Niobium Tungsten Oxide
- Capacity ~200–280 mAh/g (theoretical)
- supports high C-rates
- operates at down to -100°C
Potassium Metal Anode
- Metal anode for potassium-ion batteries, analogous to a lithium or sodium metal anode.
Sodium Metal Anode
- Metal anode for sodium-ion or sodium metal batteries, analogous to a lithium metal anode.
Silicon
- Capacity 3580 mAh/g (theoretical) ~10x the theoretical capacity of graphite
- Volume expansion/contraction during insertion/extraction (∼400%)
Tin
- Capacity 847 mAh g^-1 (theoretical)
- large volume expansion during insert/extraction
- electrolyte compatibility
Titanium Niobium Oxide (TNO)
- A fast-charging anode material positioned between graphite and lithium titanate in voltage and rate performance.

Anode Components

The anode is not just graphite or graphite and silicon. It needs additives to increase the conductivity and a binder to hold it all together.

Active material
- hard carbon
  - hard carbon-tin composite
- graphite
  - natural
  - synthetic
- graphite and silicon
- silicon
- soft carbon
Conductive additive
- Carbon Black
- Carbon Nanotubes
Binder
- Carboxymethyl Cellulose (CMC)
  - Refining Carboxymethyl Cellulose
- Styrene Butadiene Rubber (SBR)

The ratio of positive and negative electrodes in lithium graphite batteries is typically N/P = 1.08, where N and P are the mass specific capacities of the active materials of the negative electrode and positive electrode respectively.

Having a larger amount of negative electrode active material helps prevent lithium from depositing on the surface of the negative electrode. Improving cycle life and safety of the battery.

Design

Why N/P Ratio Matters in Lithium-Ion Cells

The N/P ratio refers to the ratio between the effective capacity of the anode (negative electrode, N) and the cathode (positive electrode, P). In simple terms, it compares how much lithium the anode can store versus how much the cathode can provide.

The Role of Cut-Off Voltage and Anode Material in Li-Ion Cells

The cut-off voltage is critical for balancing energy density and ensuring cell longevity and stability. While cathode chemistry often guides cell classification, variations in anode materials significantly impact the appropriate cut-off voltage setting.

In lithium-ion cell design, one of the most critical relationships shaping performance is the link between anode porosity and energy density. As energy density increases, conventional design logic suggests reducing anode porosity to pack more active material into the electrode. And up to a certain point, this is exactly what happens.

However, beyond a specific energy density threshold, this trend reverses.

Manufacturing

Electrode Manufacturing: there are 4 steps in electrode manufacturing: Mixing, Coating, Drying and Calendering. The electrode manufacturing process is the same for all of the cell chemistries and formats in terms of the basic steps. Anode: active material (eg graphite or graphite + silicon), conductive material (eg carbon black), and polymer binder (eg carboxymethyl cellulose, CMC)

Anode Overhang

The extra material in a battery cell’s anode that extends past the intended boundaries.

Testing and Validation

Before an anode formulation or coating batch is qualified for cell builds, it needs to clear physical quality checks (mass loading, thickness, adhesion, moisture content) and electrochemical validation via half-cell testing against lithium metal.