diff --git a/src/data/entities.rs b/src/data/entities.rs
index b16b6fb..abbed41 100644
--- a/src/data/entities.rs
+++ b/src/data/entities.rs
@@ -1,5 +1,5 @@
 use crate::core::srs::{FADE_PER_DAY, MAX_SCORE, WordOpenMode};
-use crate::data::{INVALID_ID, Id};
+use crate::data::{Id, SmallId, INVALID_ID};
 use crate::data_provider::history::HistoryRepositoryInner;
 use chrono::{DateTime, Local, NaiveDate, Utc};
 use fsrs::MemoryState;
@@ -19,7 +19,7 @@ pub struct WordData {
     pub key: String,
     pub value: String,
     pub tags: String,
-    pub additional: LiteMap<String, String>,
+    pub additional: LiteMap<Id, String>,
 }
 
 impl EntityData for WordData {
diff --git a/src/data_provider/words.rs b/src/data_provider/words.rs
index c82e85a..b1f25a8 100644
--- a/src/data_provider/words.rs
+++ b/src/data_provider/words.rs
@@ -1,7 +1,9 @@
+use litemap::LiteMap;
 use crate::data::entities::WordData;
 use crate::data::repositories::{DataRepository, DataRepositoryInner};
 use crate::data_provider::{SqliteDbLayer, TableAccessQuerySet};
 use rusqlite::{Error, Params, Row};
+use crate::data::{Id, SmallId};
 
 pub type WordRepository = DataRepository<WordData, WordDataLayer>;
 pub type WordRepositoryInner = DataRepositoryInner<WordData, WordDataLayer>;
@@ -32,7 +34,7 @@ impl TableAccessQuerySet<WordData> for WordQuerySet {
             key: row.get(1)?,
             value: row.get(2)?,
             tags: row.get(3)?,
-            additional: serde_json::from_str(&additional).unwrap_or_default(),
+            additional: additional_from_string(additional),
             group_id: row.get(5)?,
         })
     }
@@ -42,7 +44,7 @@ impl TableAccessQuerySet<WordData> for WordQuerySet {
             &entity.key,
             &entity.value,
             &entity.tags,
-            serde_json::to_string(&entity.additional).unwrap_or_default(),
+            additional_to_string(&entity.additional),
             entity.group_id,
         )
     }
@@ -52,8 +54,29 @@ impl TableAccessQuerySet<WordData> for WordQuerySet {
             &entity.key,
             &entity.value,
             &entity.tags,
-            serde_json::to_string(&entity.additional).unwrap_or_default(),
+            additional_to_string(&entity.additional),
             entity.id,
         )
     }
 }
+
+fn additional_to_string(map: &LiteMap<Id, String>) -> String {
+    let mut parts = vec![];
+    for (id, value) in map {
+        parts.push(id.to_string());
+        parts.push(value.clone());
+    }
+    parts.join("")
+}
+
+fn additional_from_string(data: String) -> LiteMap<Id, String> {
+    let parts = data.split('').collect::<Vec<&str>>();
+    let chunks = parts.chunks(2).collect::<Vec<&[&str]>>();
+    let mut map = LiteMap::with_capacity(parts.len());
+    for part in &chunks {
+        let id : Id = part[0].parse().unwrap();
+        let value: String = part[1].to_string();
+        map.insert(id, value);
+    }
+    map
+}